An eerie underground region centered around a natural system of caves, lit by magic and phosphorescent fungus. The natural and artificial seem to blend together: a cavern might be carpeted by fungus or straggly grass, and yet have its walls lined with bookshelves and cabinets; a building might be calcified and cut through by a milky stream inhabited by blind fish and hideous, gigantic salamanders, pale gray with tiny, reddish eyes. It is home to the terrifying Booklords.

Its sapient population is almost entirely human- if the Booklords can be counted as human- and undead; the Mountain Dwarves will not touch the place.

Most permanent entrances are located along the southwest coast of Thass, although this doesn't necessarily reflect the region's physical location.