Underground ruins (synonyms: subterranean remains; example toponyms: the Lost City of Ke, Bunker Two) are ruins that exist entirely or mostly beneath the surface of the earth. Underground ruins may be the remains of an ancient temple, underground city, crashed spaceship, library, tomb, mine, or other structure. They may include caved-in portions, old traps, mine cart tracks, triggered mechanisms, or gas-filled chambers. They may be guarded or home to subterranean creatures or civilizations (these occupants may have been the original builders or may have moved in afterwards). Often underground ruins hold treasure or ancient knowledge. There may be maps or clues that lead to their location. They may be accessed via hidden door, excavation, cave system, sewer system, or other subterranean area. Underground ruins are home to subterranean creatures, mutants, explorers, and archeologists.

When designing underground ruins consider the event that led to the structure being underground – was it built this way or did some event occur to bury it? Consider also the light sources that illuminate the space.