Exploring The Underdark
What squelches through unlighted corridors miles below the daylit surface world? What secrets of ancient vintage rest behind long-cooled barriers of solidified magma? What perils and terrors await those foolhardy enough to venture into such a dismal and dangerous environment? Adventurers from all over Faerûn dare the depths of the Underdark in search of fame, fortune, and power, but all too often they find only death - or fates even worse than death.
Geology and Environments
A cave is a natural opening in rock that is large enough for creatures to enter. The Underdark, simply put, consists of a linked network of several titanic cave systems. Large portions of it do fit the definition of "a natural opening in rock," but the Underdark also encompasses areas of deep water that hide coral caves, hollowed-out sections of ice in which creatures live, and places where fungus, bone, or even pure force form "caves."
Underdark terrain is dynamic and changeable. A map drawn a few decades ago may show tunnels that have long since collapsed, or lakes that are now dry. Terrain can change gradually over the course of decades, centuries, and millennia, or swiftly as a result of an earthquake or volcanic eruption.
The surface world is marked by mighty mountains, high plateaus, and vast plains. The Underdark possesses none of these features, but it does have physical features all its own. Unlike the surface world, the Underdark is uniquely three-dimensional. Knowing the direction of true north is not enough to navigate the Realms Below; a traveler must also know the depth underground of her destination. It is possible to find the correct coordinates but still be several miles too deep or too shallow.
Many of the Underdark's features can be explained as nothing more than the results of purely physical forces, albeit sometimes on a grand scale. Other features would be unlikely or even impossible in worlds where magic, elementals, planar interstices, and divine caprice did not help to shape the deep places.
Vast, empty voids of awesome scope, Faerûn's abysses are rare features that can form insuperable barriers to travel. An abyss is simply a great open space, sometimes many dozens of miles in breadth and virtually bottomless. Some Underdark abysses are scores of miles deep. The difference between an abyss and a vault is difficult to define, but as a general rule, a vast space approachable from its higher reaches is an abyss, while the same space approachable from the floor might be better described as a vault. Abysses tend to be larger and deeper than areas that are considered vaults, but this is not always the case.
Like the starkest mountains of the Lands Above, abysses are often completely impassable to anything without wings. Underdark civilizations near such features sometimes carve out harrowing ledge-paths to circle the tremendous void of the neighboring abyss, or endless stairs to descend its walls.
Below is some simplified cave terminology to provide a basis for description. These terms apply equally well to "caves" of any material.
Cavern: A large chamber in a cave.
Dead Cave or Dry Cave: A cave without running streams, lakes, or drips of water.
Gallery: A large, mostly horizontal chamber.
Lake: A standing body of water of any size. What upperworlders might call a puddle may be a lake in the Underdark.
Live Cave or Wet Cave: A cave with flowing water or a lake.
Maze Cave: Several passages and chambers interconnected, often on two or more levels.
Ossuary: A cave filled with bones. If the bones are fossils, the cave is often called a fossil chamber.
Spring: A natural upwelling of water from rock or soil. Few springs offer drinkable water; the majority of them are laden with harmful substances.
Perhaps the most common topographical feature of the Underdark, a cave system consists of a series of caverns and passages that may stretch for miles. Caves can be forme by several different methods, but the most common is the action of flowing water. Cave systems often twist, turn, climb, and drop in a maddening fashion, forming three-dimensional mazes that dishearten even the most determined mapmakers.
Caves vary widely in terms of their habitability. Living caves that include streams or rivers are full of life (by Underdark standards) and can often support surprisingly large populations, especially of improbably large and ferocious monsters. Other caves may be barren wastelands, without food or water.
Over the course of a hundred or more centuries, Faerûn's deep caverns and vaults have been expanded tremendously by the delving of various Underdark races. Thus, the term dungeon when applied to the Underdark means a structure excavated from the surrounding rock by intelligent creatures. For example, a great duergar city delved into the wall of a vault might be considered a large dungeon, with halls and passageways extending for miles from its entranceways. Dungeon complexes often serve to link two natural features (such as two or more vaults close to each other) with a system of artificial caves that vastly extends the scope of a natural cave system.
Dungeons come in two varieties - abandoned and occupied. Since they are not sources of food or water in and of themselves, empty dungeons do not necessarily attract Underdark settlers. However, dungeons are often supremely well suited for defense, - and a dungeon that happens to be near a rich area such as a living cave is almost certain to be occupied by something, even if the original builders are long gone.
Just as on the surface, water can carve deep canyons and gorges in the Underdark. An Underdark gorge is nothing more than a cave that runs vertically instead of horizontally. Gorges often feature streams (and therefore life and food), although the difficulty of the terrain makes a gorge less desirable as a residence than a living cave with less extreme topography.
Since gorges can run for many miles, they often serve as the highways of the Underdark. Travel along the floor of a gorge can be very difficult, but many Underdark races take steps to improve these natural roads for the use of their own merchants and hunters. Gorges also offer good opportunities to change depth and perhaps access another level quickly, through a little climbing.
Water is common in the Upperdark, since runoff from the surface frequently drains into cave systems below ground. In many areas, the water table is close enough to the surface that only the most shallow cave systems can form. However, due to the unusual factors involved in the creation of Faerûn's Underdark, a water table 20 feet below ground does not necessarily mean that air-filled caves don't exist at greater depths. Planar connections, particularly to the planes of Earth and Water, make very unlikely hydrology possible.
Any body of fresh water is called a lake. Underdark lakes range in size from small pools to inland seas hundreds of miles in extent. Large lakes typically occupy either tremendous vaults or connected networks of partially submerged eaves. The Lake of Shadows and the Giant's Chalice are examples the former type, and the Darklake is an example of the latter. If a lake has both an inlet and an outlet, its water is usually drinkable, but lakes that are not refreshed from time to time may stagnate.
Most lakes are found in the Upperdark or Middledark. Bodies of water that collect in the Lowerdark simply can't drain to any lower elevations, so they tend to be seas (brackish water) instead. However, planar connections to the Elemental Plane of Water mean that at least a few of the bodies of water in the Lowerdark hold fresh water.
Large lakes can form the best and most accessible highways of the Underdark. In many places, however, the cavern ceiling descends to meet the water, making the lake impassable to all but aquatic creatures.
Unlike gorges, rifts are not formed by erosion. Rather, they are the scars of tremendous upheavals deep in the earth. Rifts are places where vast blocks of stone rose, sunk, or slid past one another in long-ago cataclysms, leaving tremendous chasms. Rifts may be dozens or even hundreds of miles in length, and sometimes miles deep, but they are rarely very wide - most are less than a bowshot across.
Rifts sometimes break apart preexisting features such as cave systems, presenting formidable obstacles to creatures traveling through eaves. In order to continue when faced with a rift, the traveler must climb or descend to the appropriate level on the far side. Like gorges, rifts often serve as vertical highways in the Underdark, offering travelers the opportunity to change depth with little fuss.
Underdark rivers tend to be swift, violent, and tortuous in their windings. It is a rare river indeed that flows level and smooth for more than a few miles at a time before disappearing into a deep gorge or sinkhole in a fuming waterfall. Rivers are the great builders of the Underdark, the natural force that sculpts great caverns and brings lifegiving energy and food to sustain the Underdark ecology. Most rivers are surrounded by a halo of living caves, which can be valuable real estate indeed.
Perhaps the most wondrous of the Underdark's features are the vast, nighted seas of the deep earth. Seas are saltwater bodies, not fresh, and most of them are found in the Lowerdark, though Underdark seas also occur at higher spots beneath Faerûn's surface oceans. While air-filled cave systems may extend for dozens or even hundreds of miles beneath the oceans above, or form air-locked siphons of extraordinary size, these features are exceptional. Most caves beneath large bodies of saltwater are simply subterranean extensions of surface oceans.
Seas tend to form in the largest of vaults, occupying caverns large enough to be miniature worlds in their own right. Like the lakes, seas offer some of the best roads in the Underdark, and many are heavily traveled.
Sometimes natural processes form deep pits or wells in the earth. The shaft of such a structure may be carved out by water flowing straight downward in a subterranean waterfall or created by volcanic activity. Unlike a gorge or a rift, a shaft tends to be a relatively small feature (usually less than a bowshot in diameter), but it may plummet for miles straight down.
Because of their relatively small cross-sections, shafts often serve to channel air movement between disparate portions of the Underdark. In places where the conditions are extreme (for example, a shaft near a superheated magma chamber), the air movement can also be extreme. Screaming winds might roar up or down a shaft in a scouring blast that would put a hurricane to shame. Sometimes, cave systems "breathe" in conjunction with changes in the surface world above, resulting in tremendous rushes of wind in and out through shafts every day.
A tunnel is simply a passage that connects one place with another. Most are cut by creatures, though some are the results of natural movements of the earth and other forces. Underdark races often cut very ambitious tunnels to link multiple cave systems. Though such dreary passageways may be dozens of miles in length, most are only 10 or 20 feet across. Other tunnels are the work of burrowing monsters such as delvers, purple worms, and umber hulks. These "natural" tunnels may be twisting, turning mazes of intersecting passages.
Tunnels are some of the Underdark's most useful roads, but they severely restrict a traveler's options. If you don't like where a tunnel leads, you really have no choice but to go back the way you came. Tunnels also offer few hiding places for those who cannot blend in with stone, so often the only way to get away from a predator is to run - and hope you're faster.
The higher reaches of the Underdark consist of immense networks of relatively small caves, but as a traveler descends, the number of caves decreases while the size of the individual caves increases. A large cave near the surface may consist of a few dozen linked chambers, each perhaps a few hundred feet long and a few dozen feet wide. But deeper down are openings in the earth that dwarf any surface dweller's conception of a cave.
A typical vault may be 2 to 4 miles across and as much as 1 mile high. Some, however, sprawl for 10 miles or more and reach heights of 5 or 6 miles from the floor. Larger vaults often feature immense columns - huge piers of natural rock that help to buttress the soaring ceiling. Some were formed by unthinkably massive pieces of the world grinding past each other in the very dawn of time, others by the influence of the Underdark's native magic, and still others by the confluence of planar characteristics in buried planar connections. However it was formed, a vault is a world in miniature, with its own streams, lakes, hills, and plateaus all contained in a single vast cavern.
Vaults are almost always highly desirable territories, since they usually offer enough space and resources to support huge forests of fungus, moss, and other strange growths. By Underdark standards, most vaults teem with life, so it comes as no surprise that they support the most powerful and numerous of Underdark settlements. Some stories even tell of illuminated vaults, places where sun-bright crystals in the ceilings blaze with the intensity of true daylight and support green plants and surface-like fauna in abundance.
It is not universally true in Faerûn that descending 40 or miles straight down in any spot brings a traveler to magma. Volcanic activity is extremely variable in the Underdark. Isolated pools of magma seep up almost to the surface in all sorts of places without any other volcanic activity, and in other places deep tunnels and vaults support humanoid settlements at depths where magma should be all that's present. Again, planar anomalies, deific intervention, and the powerful magic of the earth itself are likely to blame. Whatever the cause of these surprising conditions, racing rivers of molten rock, caverns full of brimstone and sulfurous reek, and scalding geysers and hot springs can be found at almost any depth in the Underdark. Underdark volcanoes aren't really mountains - they are usually tremendous fissures or magma chambers that can vomit deadly rivers of lava into nearby caverns with little or no warning.
Rocks and Rock Formations
All the topographical features of the Underdark share one thing in common: They're surrounded by rock. Rock in its various forms is every bit as relevant to the denizens of the Underdark as weather is to surface folk. Understanding the significance of different sorts of rock often means the difference between life and death for those who live in the Realms Below.
Rocks are divided into four categories: sedimentary, igneous, metamorphic, and magimorphic. These labels describe how the rock was formed. These categories of rock vary widely in their hardness, as shown below.
|Sedimentary (sandstone or shale)
|Igneous (basalt, rhyolite)
|Metamorphic (gneiss, granite, or marble)
Magimorphic rock spans a wide range of hardness, since it is often derived from the other types.
Sedimentary rock formed from softer clays and deposits of organic matter long ago. These materials were compacted into discrete layers in the same sort of process that turns snow into glacial ice. Sedimentary rock is soft and easily weathered, which means that flowing water can gouge vast cave systems from it in a relatively short time, geologically speaking.
Many of the classic cave features, such as stalagmites and stalactites, are found in caves formed from sedimentary rock. Such features are called solution-based, or dripstone, formations because they form from the slow motion and evaporation of water that contains dissolved minerals.
A solution-based rock formation is considered "living" if it is still in the process of forming. With enough patience and time, the growth patterns of such formations can be shaped, although it may take dozens of years for such alterations to become evident. Some Underdark races, such as the drow and the svirfneblin, occasionally shape growing deposits into particular forms.
Stalactite: Mineral-laden water drips downward, creating a build-up of rock on the ceiling of a cavern.
Stalagmite: When mineral-heavy water drips down to the floor, the bits of rock and mineral start to pile up, creating a slowly growing protrusion on the floor.
Column: When a stalactite and a stalagmite meet and join, they're no longer referred to by their previous names. The resulting formation called a column.
Cave Pearl: Sometimes mineral-rich water drips too vigorously to form a coherent stalagmite. Where this occurs, a piece of gravel or bone might become coated with mineral deposits and polished to a round and smooth shape by the flowing water. Such an object is called a cave pearl. A well-shaped pearl with good color and texture is worth between 5 gp and 50 gp.
Cost: 25 gp (average).
Curtains, Draperies, and Blankets: These terms refer to dripstone in the form of rippled, wavy, or folded sheets. Curtains and draperies are thinner, often translucent, and resonant. Blankets are thicker so they tend to muffle sound a bit. Often these features have stripes of different colors.
Flowstone: A layer of deposited material that follows the shape of the underlying walls or floor is called flowstone. A flowstone formation often has draperies at its lower end.
Soda Straws: These thin flutes, each the width of a drop of water, are how stalactites begin.
Shield: Water flowing through a cave wall or ceiling sometimes builds up sediment on both sides of its entry point, creating two parallel plates on either side of a thin, sheetlike crack.
Popcorn: This knobby, crystalline growth forms in a wide variety of cave conditions and is sometimes called cave coral.
Boxwork: Boxwork forms when softer rock wears away, leaving a harder crystalline network of thin blades poking out from a ceiling, floor, or wall.
Crystal Flowers: These crystalline formations, also called cave flowers, are valued by several Underdark races for their aesthetic beauty and their commercial value. Crystal flowers are usually made of halite (simple table salt), gypsum, or some other minerals. The petals are fibrous or prismatic crystals that resemble growing seedlings. Halite flowers often grace the table settings of wealthy drow on special occasions. Even illithids occasionally carry salt in crystal flower form to add earthy spice to their brain meals.
Cost. 25 gp; Weight 1 lb.
Spar: This term refers to crystal growths in which the crystal faces and structure are visible to unaided sight. Many-faceted crystals that form underwater are called pool spars.
Igneous Features and Rocks
Igneous features are formed or influenced by fire - usually magma chambers and flows, although openings into the Elemental Plane of Fire are not uncommon in many parts of the Underdark.
Magma and Lava: While molten rock is underground, it is called magma. If it erupts to the surface from a volcano, it is called lava.
Hot Springs: Water heated by volcanic forces often bubbles up through the ground in hot springs. Some of these springs make it to the surface; others can be found in caverns underground. Often, the water is so laden with sulfur and other minerals that it is undrinkable.
Geyser: A geyser consists of a deep fissure or well in which water can accumulate and be heated to high temperatures and a thin shaft that leads from this pool of superheated water to the surface. Periodically, a fountain-like jet of hot water and steam erupts from the opening. Such an eruption deals 2d6 to 6d6 points of impact damage and 2d6 to 10d6 points of fire damage to any creature it hits. The intervals of eruption can be regular or irregular.
Lava Tubes: These tubes form after lava has flowed from a volcano. The surface of the lava cools and solidifies while the interior is still liquid. This uneven cooling creates a crust that resembles a tube. Lava tubes can branch and fork, following the passage of the lava. Because the cooling can happen over several intervals, it's also possible to have a tube within a tube.
Pumice: Rock that has been made liquid and frothy hardens into a light, porous stone called pumice.
Stalactites: Stalactites formed by lava happen in two main ways. Tubular lava stalactites are usually formed by volcanic gases. Gas flows through lava in which different minerals are solidifying at quicker or slower rates, forming long tubes. The downward drips from such formations may form stalagmites. The second type of lava stalactite is called a sharktooth stalactite. This formation occurs when flowing lava coats existing protrusions, resulting in a broad shape that narrows to a point.
Stalagmites: Drip stalagmites form from material that pours off of or out of tubular lava stalactites. These puddles of lava may mound up in stalagmites.
Spiderstone: Drow architects use this term to refer to obsidian, which they value for its glossy black appearance. Spiderstone is frequently used in drow buildings that feature spider motifs.
Geodes: Often found in deserts or volcanic regions, these hollow rocks are lined with crystals. In the Underdark, some geodes are immense, growing to the size of caves. One legend tells of a wizard entombed in a 40-foot geode filled with amethyst crystals.
Metamorphic rocks occur where intense heat and pressure act to harden rock and alter its consistency. Marble is a good example of a sedimentary rock (limestone) that volcanic activity has transformed into a much harder stone. Metamorphic rock formations are similar to those of the premetamorphic rock, though often compressed. Caves formed from metamorphic rock more often take the form of faults, fissures, or similar discontinuities caused by the movement of adjacent slabs of stone.
Many metallic ore deposits form in metamorphic settings. Some valuable minerals, such as garnet and tourmaline, also form only in such environments. Hence mining-oriented cultures, such as that of the dwarves, tend to settle in areas that feature metamorphic rocks.
Since metamorphic stone is often dense, hard, and beautiful, some varieties are highly regarded as building materials. Many huge quarries can be found in the depths of the Underdark.
Magimorphic (magic-formed Rocks)
Magimorphic rocks are stones or rock formations that have been warped and changed by magic. The Underdark of Faerûn includes several very different manifestations of magimorphic rocks.
Clear Black Rock: In its natural state, this rock is found only in the Lowerdark, where it is cut in slave-worked quarries and prepared for export to the Middledark and Upperdark. Clear black rock is not common, but it is certainly available to those willing to pay premium prices. To regular and low-light vision, this rock looks black, shiny, and perfectly opaque, but to darkvision, it is perfectly clear. Creatures without darkvision often mistake it for obsidian, but drow, mind flayers, and other creatures gifted with darkvision find it a challenging yet worthwhile stone to incorporate into their strongholds and encampments.
Crumblestone: This rock breaks very easily and is the bane of miners and travelers anywhere. Crumblestone exists in a precarious equilibrium with the other rocks or strata surrounding it. A Small or larger climbing or flying creature that begins to put its weight onto crumblestone must make a DC 15 Balance check or fall when the rock beneath it turns to powder. For flying creatures, this situation rarely poses a problem, but for climbers, an encounter with crumblestone is often deadly. If enough crumblestone gives way in a cavern, a cave-in becomes likely.
Crysstone: Beautiful, intricate, and very delicate, crysstone is a rock that resembles spun glass. It is very hard but not at all durable (hardness 8, 2 hit points per inch of thickness). Crysstone shatters easily, and sonic damage automatically bypasses its hardness: Because it is so susceptible to sonic damage, the threat posed by monsters with Wide-area sonic attacks increases in areas where crysstone is prevalent.
Darkstone: Darkstone seems to drink in light. In tunnels cut through this material, light sources dim, shedding a glow only half as strong as usual. Thus, a bullseye lantern illuminates a cone only 30 feet long and 10 feet wide, while a torch lights only a 10-foot radius. A daylight spell in a darkstone area radiates daylight in only a 30-foot radius.
Photogenerative Rock: These rocks grow quickly when exposed to light. In each round of exposure, a photogenerative rock doubles in size until it's sixteen times as large as it was when unlit (4 rounds). Eliminating the light halts its growth, and each hour of darkness reverses the effect of 1 round of light. Some Underdark undead use photogenerative rock in their demesnes to stall light-bearing adventurers until the undead can ready their defenses. Photogenerative rock can also be used to seal a doorway and divert light-bearers into more trap-ridden or better-defended areas.
Photostatic Rock: Events that happen in the presence of this rock imprint upon it for a short time. One cubic foot of photostatic rock picks up impressions in a 10-foot radius and automatically relays them to anyone who touches it later. The effect is similar to that of a stone tell spell, except that the photostatic property reveals only what happened within its radius in the past hour. Every additional cubic foot of photostatic rock provides 1 more hour of memory and expands the radius of sensitivity by 10 feet. A photostatic rock records only what it witnesses (treat its perspective like a burst), so it cannot record what goes on beyond a closed door.
Quickstone: Quickstone is the Underdark's answer to quicksand. Like many Underdark features, it's much more terrible than its surface-world equivalent. Quickstone looks like solid stone and blends into the surrounding rocks, but it functions like quicksand except that the Difficulty Class for each maneuver increases by 5. Transmute mud to rock permanently solidifies quickstone, but a success on the spell's Reflex saving throw enables a creature trapped within it to escape.
Rock Gourds: These rocks have been warped by long-term proximity to a portal leading to the Elemental Plane of Water. Rock gourds are rare, naturally occurring stones, not created magic items, though they register as faint conjuration magic under examination with a detect magic spell. Shaking a rock gourd causes water to dribble out. The ability of these stones to produce up to a gallon of water per day apiece makes them highly valued commodities in the Middledark and Lowerdark regions, where water is scarce.
Cost: 500 gp. Weight 10 lb.
Sickstone: Sickstone glows with a nauseating, not-quite-green, not-quite-silver light that provides illumination to a radius of 40 feet. Any creature within the range of this illumination must make a DC 15 Fortitude saving throw each round or take 1d6 points of Constitution damage. A successful save reduces the Constitution damage to 1 point. Anyone in physical contact with sickstone takes a -4 penalty on this saving throw, and any Constitution damage it takes becomes Constitution drain instead. The damage caused by sickstone results from a magical disease, so creatures immune to magical diseases are immune to the effects of sickstone.
Slickstone: This rock is very smooth to the touch. Although it is natural stone, it functions at all times as if it had a grease spell cast upon it. An actual grease spell cast upon slickstone has no additional effect.
Only a newcomer to the Underdark would be naive enough to think that Toril's deep caverns are formed exclusively of rock. A variety of other materials can form the same sorts of features.
Coral: This "living rock" can be found in many saltwater-filled caves. Large clumps of coral often have tunnellike spaces winding through them, and aquatic creatures such as tritons sometimes use this substance as an architectural medium, building complex structures by forcing it to grow in particular patterns.
Ice Cave: This term refers to a cave where either seasonal or permanent ice can be found. Since there's little drinkable water in the Underdark, caves containing ide often attract settlers or predators.
Glacial Cave: A glacial cave is one formed entirely of ice. Such a cave provides more potential water than an ice cave, but also more potential danger. Fire and other heat sources (even body heat) might be enough to weaken or fracture the ice and start an avalanche.
Bones: Some areas of the Underdark have a surfeit of bones and remains of the dead. Skeletons range in size from Diminutive mice to Colossal behemoths. Moving through a section piled with skeletons means creeping or breaking through tunnels made of bones.
Force: Permanent walls of force are costly to create. Nevertheless, many of them exist in the Underdark. Some block off the territory of one race from another, others act as bridges across deep chasms, and still others serve no discernable purpose.
Fungi: Many caverns in the Underdark are full of fungus, much of it innocuous. Rather than expend the resources to hack away all of these fungi, denizens often just create routes between or through large patches of it. Tunnels cut out of mushroomlike material are not unusual sights in the Underdark.
Other Cave Features
Some caves are formed and shaped by eroding winds and freely flowing water, such as streams or ocean currents. The following features are common in such environments.
Wind: Strong or constant winds gradually shape caves and caverns. Wind effects may be present in caves that are close to the surface world, portals to the Elemental Plane of Air, or the hot gases emitted by volcanic activity. Wind-shaped caves and caverns are sometimes referred to as Aeolian caves.
Air Pockets: Subterranean waterways often have air above them, but the amounts vary. A lake might be housed in a vast cavern, but a swiftly moving river might have only one or two places where a swimmer might catch a gulp of air. Even more dangerous are pockets of gases other than air.
River Canyons: Regional uplifting of tectonic plates causes rivers to cut deep canyons. Sometimes these gorges provide surface-world entry points to the Underdark.
Subterranean rivers also create canyons. Such canyons might or might not have rivers at their bottoms now, since underground rivers often dry up or change course.
Swirlhole: A swirlhole is a generally circular hole in the rock of a streambed, eroded by water eddies and rubbed smooth by pebbles.
Halocline: The area where fresh water (such as that from a river) and salty ocean water meet is cloudy and called a halodine. Such a region often marks territorial boundaries between aquatic races.
Sea Caves: Seas and oceans are powerful forces that can wear the hardest of rocks down into fine sand. Tides and waterflow continuously redistribute this material.
Tufa Caves: Springs, rivers, and the sea can deposit a porous limestone called tufa. Finding this rock indicates that water is (or at least was) nearby.
The Underdark Environment
Walking through wild caves without end is different than stalking monsters in a dungeon near the surface. Travelers venturing into the Underdark enter a world in which nothing can be taken for granted. Vast portions of the Realms Below are wastelands devoid of food, water, and even light.
Except for any lanterns that characters bring into the caves, the Underdark is absolutely dark. Coupled with the tomblike silence of most passages, absolute darkness can unnerve even the most experienced caver. Thus, extra lights and oil (or magical means of illumination) are important for any cave trip.
Halflings, humans, and other surface dwellers without darkvision must absolutely carry some form of light with them in order to navigate the Underdark. While torches or lanterns are feasible for relatively short trips, a journey of more than a few days might require dozens of torches and oil flasks, making non-magical light impractical for long expeditions.
The best and most efficient means of providing light in the Underdark is to use minor alchemical or magical items, such as sunrods or continualflame torches. A sunrod is small, light, and long lasting, capable of providing illumination in a 30-foot radius for 6 hours. A party of explorers could easily carry several dozen sunrods, which would provide enough light for many days of travel. At 2 gp apiece, an investment of 100 gp or so is well worth the money.
The continualflame torch is potentially cheaper, since the material component costs only 50 gp and the item lasts forever if not destroyed. However, such a torch only illuminates a 20-foot radius.
While most of the Underdark is exactly that, some locales possess sources of natural illumination.
Luminescent Growths: Rare forms of lichen, fungi, and moss are bioluminescent. In sufficient quantities, these growths can illuminate large caverns. Most bioluminescent growths are quite dim, so a cavern illuminated by glowing moss or fungi is typically as dark as starlight, although some particularly bright regions might be equal to moonlight.
Molten Rock: Areas with exposed pools or streams of magma are illuminated by the dim, ruddy glow of the molten rock. The glare of molten rock is typically equal to moonlight.
Radiant Crystal: Some rare rocks of the Underdark are naturally radiant, ranging in brightness from starlight to full daylight, although daylight equivalence is quite rare. The great vault of Deep Imaskar is roofed with radiant crystal that is as bright as weak surface daylight.
Reflective Stone: While not naturally luminescent, caverns made partially of reflective stone can be much more easily illuminated by small light sources than normal. Reflective stone quadruples the radius of illumination of any light source brought inside.
Wizard Fire: The rarest and most wondrous of natural illuminations in the Realms Below, wizard fire consists of dancing sheets of dim light, like the northern lights of the surface world. Though it is only as bright as starlight, wizard fire is weird and beautiful.
|2d4 x 10 ft.
|3d6 x 10 ft.
|Gorge or shaft
|6d6 x 10 ft.
|6d6 x 10 ft.
|Rift, tunnel, or abyss
|Limit of sight**
|*If one party is carrying light equal to torches or brighter, double the encounter distance for the other party.
**A light source can be seen at a distance equal to 20 times its radius of illumination.
Seeing Distant Light
Carrying lights can be quite dangerous in the Upperdark. In a large cavern, a sunrod or torch can be spotted from much farther away than its radius of illumination, which means that any creatures nearby have plenty of time to decide whether to avoid the surface dwellers or lay an ambush for them.
Complete Darkness: In general, a light source can be spotted (Spot DC 20) at a distance equal to 20 times its radius of illumination, if the area is otherwise in complete darkness. For example, a sunrod can be seen from 600 feet away, provided that nothing obstructs the line of sight. An observer who fails this Spot check automatically spots the light source at half that distance.
Dim Light: In conditions equal to starlight or moonlight, a light source can be spotted (Spot DC 20) at a distance equal to 10 times its radius of illumination. For instance, a sunrod can be seen from, 300 feet away in these conditions. An observer who fails this Spot check automatically spots the light source at half that distance.
Using Distant Illumination: Creatures. outside the illumination of a light source can see into it just fine. An observer who is close enough to spot the light source automatically (10 times the radius of illumination in complete darkness, or half that in dimlight) can make Spot checks as normal to discern creatures or objects in the illuminated area.
In general, the Underdark is surprisingly well ventilated. Vast subterranean spaces and the rare planar connection to the
Elemental Plane of Air provide plenty of good air for living creatures. However, this is not universally true.
Sealed chambers, isolated caves, and water-isolated caverns have finite supplies of good air. Creatures that don't breathe need not concern themselves with air quality, but other travelers do.
Any enclosed space that is reasonably airtight can quickly become stale or depleted. In general, a Medium creature depletes about 1,000 cubic feet of air (a 10-foot cube) every 6 hours. A Small creature requires one-half as much air, and a Large creature four times as much. Air becomes stale when it is halfway to depletion.
Stale Air: Regions that are poorly ventilated are not immediately deadly, but the stale air can quickly wear out even the hardiest travelers. A character in stale air must succeed on a Fortitude save each hour (DC 15, +1 per previous check) or become fatigued. A fatigued character must succeed on a Fortitude save each hour (DC 15, +1 per previous check) or become exhausted. An exhausted character takes 1d6 points of nonlethal damage every 15 minutes until the condition is corrected. Characters cannot recover from their fatigue or exhaustion until they reach an area of good air.
Depleted Air: Depleted areas can prove deadly in a matter of minutes. See Slow Suffocation, page 304, DMG.
Gases and Fumes
Some areas of the Underdark are plagued by air that is not just stale, but actively toxic. Often this situation results from geothermal activity in the vicinity of volcanoes. Sometimes the escaping gases emerge with an audible hiss or a putrid smell, but not always.
Gases tend to concentrate in areas that are isolated in some way from the nearby passages. For example, a passage that dips down sharply and then climbs up again forms a natural, low-lying pocket where deadly fumes can accumulate. An-air-filled passage sealed by water siphons at either end could also concentrate deadly gases.
Irritating Fumes: These gases cause coughing, stinging of the eyes, dizziness, and similar difficulties. A character exposed to irritating fumes must succeed on a Fortitude save once per minute (DC 15, +1 per previous check) or become sickened. Recovery from this condition is not possible until the character leaves the affected area.
Poison Gas: These gases are deadly. Anyone who ventures into an area of poison gas must succeed on a Fortitude save once per minute (DC 13, +1 per previous check) or take 1d4 points of Constitution damage.
Some poisonous gases also have the characteristics of irritating fumes, so their presence is obvious. Others, however, offer no telltale burning of the throat or stinging in the eyes to warn creatures of the danger. Any creature exposed to poison gas is entitled to a DC 15 Survival check to detect the threat before breathing enough to force a saving throw. With a successful check, the creature can retreat before risking any damage. A creature with the scent ability gains a +5 bonus on this check.
Explosive Gas: Some naturally occurring gases can explode in the presence of open flames. If a burning torch or lit lantern is brought into a pocket of explosive gas, the vapor explodes, dealing 3d6 points of damage to each creature in a 10-foot radius (Reflex DC 15 half). An alert spelunker might notice the danger before causing an explosion, since the open flame often behaves strangely (burning in a different color or exceptionally brightly) right before the explosion. Allow the creature carrying the light a DC 20 Survival check to observe the danger before the explosion actually occurs. If an open flame remains in the hazardous area thereafter, there is a 50% chance of an explosion in each subsequent round.
The temperature underground varies depending on the depth and the presence of geothermal features, but the vast majority of the Underdark has a moderate temperature throughout the year. It tends to be on the clammy and chilly side, but typical adventuring garb is sufficient to keep characters warm and dry.
Cold Water: Water is almost always dangerously cold in the Underdark, since it is never warmed by sunlight. A character who becomes soaked in cold water is much more vulnerable to cold conditions than a dry character. Moderate temperatures are treated as cold temperatures for the purpose of determining whether the soaked character suffers cold damage (see Cold Dangers). This vulnerability remains for 2d4 hours or until the character changes into dry clothes; soaked clothing provides no protection.
A character who remains immersed in cold water for more than 1 minute takes 1d6 points of nonlethal damage per additional minute of immersion.
Geothermal Heat: Caverns heated by geothermal activity can be quite warm; in fact, hot or severe temperatures are common near volcanoes. Any chamber with exposed magma is severely hot, and possibly extremely hot (see Heat Dangers).
Besides its many predatory races and monsters, the Underdark is also home to a variety of natural animals that live out their lives normally below the ground. These animals include (but are not limited to) bats, crickets (and cricket droppings and eggs), eyeless cavefish and crayfish, springfish, salamanders, rats, flies and gnats, and spiders. Additionally, it is not unusual for creatures in the Upperdark to encounter the occasional raccoon, frog, stray dog, or even lost dwarf, elf, or other representative of a surface race who has accidentally fallen down a deep shaft or well.
Plants and Fungi
Plants of one kind or another are the beginning of any food chain. By organizing inorganic minerals and capturing the energy of sunlight, plants create food that animals of all kinds depend on. Since plants in the Underdark do not have access to sunlight, they must make food by other means. Thus, most take very different forms than the green plants of the surface world.
Most of the Underdark's plant life consists of a tremendous variety of fungi. Fungus normally requires some amount of detritus or decaying material to thrive. So where does the fungus find its food? The answer is simple: magic. The natural magical radiation of the Underdark and its various planar connections support many weird fungal growths, as well as lichens, mosses, and other simple plants, whose existence would otherwise be impossible. In effect, faerzress is the sunlight of the Underdark, forming the basis of the subterranean food chain. Underdark regions particularly rich in faerzress or planar energies have been known to support fantastic forests of pale, gnarled trees or crystalline plants. These growths are completely adapted to their lightless, hostile environment.
Surprisingly, however, green plants are not entirely absent from the Underdark. Some caverns illuminated by particularly bright radiant crystals can actually support green plants. Caves with this sort of dazzling illumination might be filled with grass; moss, ferns, creepers, or even small trees. Any such place is a treasure beyond price in the Underdark, and it is certain to be guarded by deadly spells, monstrous guardians, or both.
Barrelstalk: Stout as a hogshead of ale, the barrelstalk is a large, cask-shaped fungus that grows up to 8 feet in height and 5 feet in diameter. Its outer layers are tough and woody, but its inner flesh is edible, and its center is filled with a reservoir of water (usually from 20 to 50 gallons) that can be tapped and drained. The inner flesh turns black and poisonous when barrelstalk begins producing spores, which happens after ten years of growth.
Bluecap: The grain of the Underdark, bluecap fungus is inedible to humanoids, but its spores can be ground to make a nutritious, if bland, flour. Bread made from bluecap flour is usually known as sporebread. Bluecap seems to do well with or without faerzress, and most Underdark humanoids cultivate it.
Cave Moss: Found only in faerzress-rich regions, cave moss is inedible to humanoids, but it is a favorite grazing food of some giant vermin, as well as rothé.
Fire Lichen: Pale orange-white in color, fire lichen thrives on warmth, so it grows in regions of geothermal heat or near connections to the Elemental Plane of Fire. Fire lichen can be ground and fermented into a hot, spicy paste, which is often spread on sporebread to give it flavor. Duergar ferment fire lichen into a fiercely hot liquor.
Luurden: Luurden, or bloodfruit, is a rare tree that grows only in areas of strong faerzress. The barren branches of this pale, gnarled tree seem more dead than alive, but once every to 4 years, it produces a small amount of bitter red fruit that is used to make rare Underdark wines and elixirs.
Ripplebark: A shelflike fungus that resembles nothing so much as a mass of rotting flesh, ripplebark is surprisingly edible without any special preparation, although it tastes much better if cooked properly. Ripplebark grows naturally in living caves.
Sussur: Rare and magical, the so-called "deeproot" tree is found only in the largest of caverns. It can grow to a height of 60 feet, and its branches are long and gnarled, with banyanlike aerial roots. Few leaves grow on the sussur; it exists almost entirely on faerzress and is often found in caverns where wizard fire is prevalent, a sussur tree can drink in magic from its environs, so most sussurs are surrounded by antimagic fields that extend for hundreds of feet.
Waterorb: This bulbous fungus is aquatic. It grows in boulder-like patches underwater wherever the water deposits detritus.
Zurkhwood: This giant mushroom can reach a height of 30 to 40 feet. Its large spores are edible with proper preparation, but zurkhwood is important primarily because its stalks are hard and woody. Zurkhwood is one of the very few sources of timber (or anything like it) in the Underdark, and many items that would be crafted from wood in the surface world are fashioned from zurkhwood in the Realms Below.
These non-magic mushrooms have combustible, long-burning caps. The caps are hard to light, usually taking 1d4+1 minutes to ignite, but once it is lit, a torchstalk. burns steadily for 24 hours and sheds light in a 10-foot radius. These fungi grow wild in the Middledark, especially in the North, and large domesticated crops of them can be found in Gatchorof, Gracklstugh, and most dwarf settlements. Drow rarely use them because they tend to prefer magical light sources when light is needed.
One torchstalk subspecies explodes into choking spores When lit. After burning for 3 rounds, a torchstalk of this variety bursts into fiery that can choke unwary travelers. Anyone in a 20-foot radius of a torchstalk when it bursts must make a DC 15 Fortitude save or take 1d4 points of Constitution damage. A DC 18 Knowledge (Underdark local) or Survival check allows an observer to distinguish explosive torchstalks from regular ones.
The Underdark supports a surprising variety of animals. A few herbivores exist there, but most are predatory in the extreme.
Many mundane hunting animals, including bears, lions, bats, rats, and monsters of all sorts, can be found in cozy underground lairs near the surface. In deeper places, animals must shift from the surface world ecology to the Underdark ecology. Many surface creatures are ill suited for such shifts and therefore cannot be found more than a few hundred feet from a cave mouth, except for places where a surface ecology is somehow maintained in the Underdark.
Bats: Better adapted to a life in darkness than most other animals, bats are extremely common in the Upperdark and Middledark. Only the lowest, most inhospitable reaches are free of these creatures, and even then, monstrous varieties such as dire bats and deep bats flourish. Near the surface, these creatures are simply surface-world bats that lair in Underdark caverns. Titanic roosts that house many thousands of such creatures can be found in some spots. Lower down, most Underdark bats are fungivores or insectivores.
Fish: Many of the rivers, lakes, and seas of the Underdark are filled with cave fish. For the most part, such fish are small, pale, relatively inoffensive creatures. Most are blind, though some that reside in illuminated caverns may retain their eyes. In the larger bodies of water, subterranean versions of dangerous fish such as sharks may be found. Cave fish need something to eat, of course, so isolated lakes don't support cave fish populations unless they're large enough to support plant life that can survive in absolute darkness.
Lizards: The Underdark is home to a variety of lizards, ranging from the poisonous spitting crawler to the wild varieties of pack lizards and riding lizards. Some are fungivores; others are dangerous hunters that can easily make a meal out of a human. Domesticated giant lizards are commonly used as beasts of burden and mounts in drow and duergar settlements.
Rothé: These grazing, muskoxlike creatures are well adapted Lfor life in the depths. Subsisting on fungi, moss, lichen, and almost anything else that grows in the Underdark, rothé are highly valued by most Underdark races and often kept in large corral-caverns.
Vermin: Perhaps the most common of all Underdark creatures are vermin. The versions native to the Realms Below range from mundane creatures the size of a mite to Gargantuan spiders and centipedes. Many, such as giant beetles and cave crickets, are fungivores, but varieties of deadly hunting vermin such as spiders and scorpions also infest the depths. The Underdark races keep some of the edible ones (mostly beetles or crickets) as livestock of a sort, but few vermin are palatable, and the giant sort are simply too dangerous to keep.
Explorers routinely encounter common dungeon hazards such as yellow mold or green slime in the Underdark. In addition, the Realms Below are home to a variety of unique perils and diseases that have laid low more than one heroic adventurer.
Underdark Slimes, Molds, Fungi, and Lichen
No place in Toril, or perhaps even the multiverse, boasts a greater variety and quantity of fungi than the Underdark. Much of it is innocuous, and some is even cultivated for food (see Plants and Fungi, above), but some of it is exceedingly dangerous.
Fool's Water (CR 3): This slime looks like a small spring of water, but a DC 20 Knowledge (dungeoneering or nature) check can reveal its true nature upon sight. Any creature that touches fool's water takes 1d6 points of acid damage. Thereafter, the substance clings to the victim like slime, dealing the same amount of acid damage each round for the next 2d6 rounds. On the first round of contact, it can be scraped off easily, but after that it must be frozen, burned, or cut away. (All these processes apply damage to the victim as well.) Against wood or metal, fool's water deals 1d6 points of acid damage but has no continuing effect.
Fool's water is extremely reactive with regular water. Trying to wash the slime off with water causes an explosion that deals 3d6 points of acid damage to every creature within 10 feet.
Extreme cold or heat, sunlight, or a cure disease spell destroys a patch of fool's water.
Cave Creeper (CR 1): Cave creeper is a gray-and-white fungus that flourishes near water. It is especially common in the Lowerdark where it grows close to most of the water sources. Cave creeper continually emits spores that cause cave terrors. Every creature within 20 feet of it must make a DC 15 Fortitude save or suffer the effect of a confusion spell. Another DC 15 Fortitude save is required 1 minute later - even by those who succeeded on the first save - to avoid taking 2d4 points of Intelligence damage. Sunlight or acid instantly destroys cave creeper.
Fire Fungus (CR 4): This fungal growth sheds a much-appreciated warmth, raising the temperature within 30 feet of it by 10 degrees. However, any open flame brought within 40 feet of fire fungus causes it to explode, dealing 5d6 points of fire damage to each creature in a 20-foot radius. Such an explosion kills the fire fungus, and it can also be killed by cold damage - 10 points of cold damage is sufficient to kill a 1-foot-square patch.
Some Underdark races use fire fungus for warmth instead of fires, since kindling and firewood are scarce in the Realms Below. Grimlocks and gloamings especially favor it.
Wisp Lichen (CR 4): This white lichen clings to the ceilings of high caverns and poses a threat primarily to flying creatures. Its sticky strands can paralyze a creature that touches them (Fort DC 11) and hold smaller creatures (up to 100 pounds) that it has already paralyzed for eventual, slow consumption (1 point of Constitution drain per hour). Larger paralyzed creatures fall if they had been flying by natural means. Because of this phenomenon, earth-bound predators usually lurk near patches of wisp lichen, waiting for the crash or thud that indicates a tasty morsel of paralyzed prey has fallen to earth.
The Underdark environment also poses a considerable threat in terms of disease. All the diseases described in the Dungeon Master's Guide exist in the Underdark, but a few diseases are unique to this realm.
Lungrot: This disease most often strikes air-breathing creatures that have been sealed inside ancient ruins.
Scaleflake: This disease most often strikes aquatic or reptilian creatures, but anyone swimming, wading, or submerged in tainted water can contract it. Scaleflake manifests as swelling, oozing, smelly pustules coating the surface of the afflicted creature's body.
Softpox: This disease makes the skin soft, swollen, and tender. This effect reduces the victim's natural armor bonus (if greater than +0) by 1 point per day. Once the creature's natural armor bonus reaches +0 (whether by reduction or because the creature had no natural armor bonus in the first place), the disease deals 10 hit points of damage each day.
|1d4 Con, 1d3 Str*
|-1 to natural armor bonus (or if +0, 10 hp damage)
|*Successful saves do not allow the character to recover. Only magical healing can save the character.
When traveling in cramped natural tunnels and through narrow, low corridors that vary widely in diameter, it is difficult to move as quickly or fight as efficiently as normal.
Many natural caves are extremely difficult to navigate. Narrow fissures, corkscrew passages, and low ceilings are common obstacles in the limestone caves of the Underdark. The table Tunnel Constriction summarizes the information related below.
Any creature fighting in a cramped space loses its Dexterity bonus (if any) to Armor Class. Beyond that, the following adjustments apply, depending upon the prevailing conditions.
Narrow or Low: An area that is smaller horizontally than the creature's space or smaller vertically than its height falls into this category, so long as the constricted dimension is at least one-half the creature's space or height, respectively. A creature in such a space moves at one-half its normal speed because of the cramped conditions, and running and charging are impossible. The cramped creature takes a -2 circumstance penalty on attack rolls with light weapons and a -4 circumstance penalty on attack rolls with one-handed weapons. It cannot use two-handed weapons at all.
Narrow and Low: An area that is smaller in both the horizontal and vertical dimensions than the creature's space falls\ into this category, so long, as each of the constricted dimensions is at least one-half the creature's space or height, respectively. A creature in these conditions moves at one-quarter normal speed and takes attack penalties equal to twice those above.
Crawl-Navigable: An area less than one-half but at least one-quarter of the creature's height is crawl-navigable. The creature can move through such a space by falling prone and crawling at a speed of 5 feet, but it must remove medium and heavy armor, backpacks, and other bulky equipment (although these can be dragged along behind). The normal penalties for lying prone apply. The only ranged weapon a creature in a crawl-navigable space can use a crossbow.
Awkward Space: An awkward space is narrower than narrow, lower than low, or smaller, than crawl-navigable, but not quite a tight squeeze. For example, a fissure 2 feet wide is less than one-half the space of a Medium creature and is therefore more cramped than a narrow space, but any Medium creature should be able to wriggle through with a little work. A creature in such a space can move 5 feet with a DC 15 Escape Artist check. Fighting in an awkward space is possible only with light weapons, and the creature takes a -8 circumstance penalty on its attack rolls. The only ranged weapon a creature in an awkward space can use is a crossbow.
Tight Squeeze: A tight squeeze is an area larger, than the creature's head but smaller than its shoulders, as described in the Escape Artist skill description. The creature can move 5 feet with a DC 30 Escape Artist check. Fighting in a tight squeeze is impossible.
Weapons In Cramped Spaces
Some weapons are, more suitable for limited space than others. Piercing weapons that jab rather than slice at an opponent are treated as one size category smaller than normal for the purpose of calculating the penalty on attack rolls. Such weapons include the dagger, short sword, rapier, spear (any type), and trident, but not the pick, scythe, gnome hooked hammer, or spiked chain.
Attacks with ranged weapons take penalties appropriate to their sizes in narrow or low conditions. Crossbows are the only ranged weapons usable in crawl-navigable and awkward spaces, but attacks made with it still take size-appropriate penalties.
|Attack Penalty Light Weapon**
|Attack Penalty One-handed Weapon**
|Attack Penalty Two-Handed Weapon**
|Narrow or low
|Narrow and low
|5 ft. only
|Escape Artist (DC 15) 5 ft.
|Escape Artist (DC 30) 5 ft.
|*A creature moving through cramped quarters of any constriction loses its Dexterity bonus (if any) to Armor Class.
**Treat piercing weapons that are jabbed at the target as one size category smaller.
In addition to tight, cramped spaces, the Underdark boasts immense caverns and huge, dark areas of space. Climbing upward is a long, arduous physical process best accomplished with spells such as spider climb or avoided altogether with fly. If magic is not an option, then a climber's kit is an invaluable tool.
Climbing down an open area can be accomplished most quickly by rappelling. To rappel, a character must have a rope and have at least one rank in either Climb or Use Rope. The character must declare the distance she intends to cover, then make a successful Climb check and a successful Use Rope check according to the parameters below.
Climb Check: While rappelling, the Difficulty Class to climb down a wall of any texture, even one that is perfectly smooth, flat, and vertical, becomes DC 10.
Use Rope Check: A rappeller can descend at her base land speed with a DC 10 Use Rope check, or she can take a full-round action to move twice her speed. If she makes a DC 20 Use Rope check, she can take a full-round action to descend at four times her base speed.
Failing a Check: A rappeller who fails either her Climb check or her Use Rope check still descends her declared distance but may go into an uncontrolled fall. To prevent this outcome, the rappeller can attempt another Use Rope check (DC previous DC + 5). On a success, she takes 1d6 points of damage but halts her movement; on a failure, she falls. If she doesn't hit bottom in the next round, she can try to arrest herself again with another Use Rope check (DC previous DC + 10). Success means she takes 3d6 points of damage; failure means she continues to fall.
If a character rappels off the end of a rope (for example, trying to descend 60 feet on a 50-foot rope) she falls from the point at which she left the rope.
Sometimes simply moving through wild caves and unworked tunnels is a challenge. To account for this, an area description similar to the following can be added to an Underdark area:
Access: The fissure leading to this cavern is an awkward squeeze (Escape Artist DC 15).
Entries of this sort describe special natural features of a wild cave and include a mechanic for navigating the space. Although many passages allow the caver to walk upright, it is also not uncommon for a traveler to have to climb, squirm, crawl, and sometimes even swim to get from place to place in a wild cave. Heroes in wild caves are subject to all these possibilities.
In most cases of limited access, a successful Climb, Escape Artist, Jump, or Swim check is required to enter or move through the area. The DC for this spelunking-specific check ranges from 10 to 20. Normal failure indicates that 1d4 additional minutes must be spent navigating the area. Failure by 10 or more indicates that a hero has become wedged into a crevice, has fallen from a height (DM applies appropriate falling damage, based on the area description), or has had some other mishap appropriate to the failed skill check.
It's not possible to get lost in a miles-long tunnel that has no branches, but sailing across a subterranean sea, or clambering through a twisting, turning mazelike cave offers plenty of opportunities for becoming completely lost.
For basic information on getting lost, refer to Getting Lost. The Underdark combines both poor visibility and difficult terrain, so any time would-be explorers venture away from clearly marked trails (or exclusionary options such as a single left-or-right branch), they may become lost. When such a possibility exists, the character leading the way must succeed on a Survival check or become lost.
|Gorge or rift
|Lake or sea
A character with at least 5 ranks in Knowledge (dungeoneering) or Knowledge (Underdark local) gains a +2 bonus on this check. A check must be made once for every hour spent in local or overland movement to see if the travelers have become lost.