Ruins of a lost city
Top level is desert theme, and may have vaults. Second floor is cavern type. Third floor is special, and contains the Old Psi-Machine, which gives the quest Mystery of the Yetis, the replacement for escorts in Embers of Rage.
NPCs are sandworms, ritches, snakes, fire and multi-hued drakes, horrors, and ghosts. The Guardian Psi-Ghost on the third floor is not hostile to you.
The schematic for Payload is on the second floor.