Underground cities and towns (synonyms: subterranean settlements, underground districts; example toponyms: Lower Grandbell, the Undernest) are settlements such as towns or cities (or portions of that location) that are subterranean. They may be the remains of an ancient city that was built on top of or may be an active component of a major metropolis. They may include underground streets and tunnels linking various buildings and areas. These interconnected buildings may be above ground structures with connected basements or may be entirely subterranean. Underground cities and towns may include transportation systems such as subway systems, may connect to the sewers, or may connect to ancient underground ruins. Ventilation ducts are a necessity. Underground cities and towns are home to mutants, rats, and people trying to avoid the surface weather.

Underground cities are common elements in fantasy worlds and science fiction worlds.

When designing an underground city, consider first if it acts alone or as a portion of an above ground location. Often worldbuilders will draw a contrast between an above ground city and its’ subterranean counterpart with the latter being home to monsters, mutants, or the slums. Such a dichotomy is far from required, however, and the underground may simply be a natural extension of the surface community. Also consider the reasons for which an underground was established. It may have provided safety and shelter from an above ground threat, it may have been established because of mining activities, or it may exist because the town is populated by creatures that simply prefer the underground.