City sublevels (synonyms: lower levels; example toponyms: the Undercity) are subterranean areas of a large megacity. These underground locations may be located deep below the surface and may contain the city slums or deep mines. They may be connected to the surface via elevators or large central shafts that provide an open conduit from the surface to these lower levels. Certain sublevels may be in ruins, taken over by gangs or criminals, or may be infested. There may be doors, gates, or checkpoints that seal these sections off. Travel between levels or to the surface may be restricted or require travel documents. Sublevels may be flooded, collapsed, or may frequently loose power. Sublevels may house maintenance rooms, maintenance corridors, as well as power systems, sewer systems, and other infrastructure critical to the city above.

City sublevels are often used by worldbuilders to explore themes of social hierarchy with the rich and powerful inhabiting the surface and the poor housed within the depths of the planet below.