Beneath the Great Rift

From publisher blurb: Investigating a newly-uncovered complex of caves in the side of the Great Rift, you and your companions discover a hive of the mysterious Derro. Now you have discovered that the cave is merely an opening to a vast world beneath the surface of the earth, and an expedition must be undertaken to both explore this dark realm, as well as to halt whatever sinister plans the Derro have for the surface dwellers. This module was written with the ADVENTURES DARK AND DEEP game in mind, but is compatible with most old-school role-playing games.