The Lost City (B4) is a Dungeons & Dragons adventure module by 1981 Basic creator Tom Moldvay. It also works fine with modern retroclones like Basic Fantasy RPG.
It was first published by TSR in 1982. The working title for the module was "The Lost City of Cynidicea." Moldvay designed the module as a low-level scenario to give novice Dungeon Masters experience in fleshing out adventures such that it is only partially complete. The plot involves the player characters discovering a ruined subterranean city slowly rising out of the sands.