Who Made the Great Rift?
Some legends say the Rift was formed when a huge cavern or series of natural caverns was torn by the fell sorceries of warring drow. Others claim that the collapse was caused by the divine wrath of dwarven deities, titanic hammerblows that buried drow. Still others say the collapse was caused by alhoon (illithid liches) magically summoning too-large dragons to defend themselves in battle against beholders, or by a dragon hiding in the caverns whose entry shaft was discovered by a rival wyrm, leading to a rock-shattering struggle.
Whatever the Rift's origin, it bears no evidence of volcanic activity or raging watercourses, the common causes of such a feature.