Sunken cities are fictional cities that have become submerged beneath the surface of an ocean, river, or large lake. Fictional sunken cities may be told about in myths or may truly exist within a fictional world. There may be quests to find these locations. Sunken cities may contain ruins, sunken statues, underwater civilizations, and shipwrecks. They may be located in coral reefs, kelp forests, or oceanic trenches. There may be whirlpools or treacherous waters surrounding them. Often in fictional worldbuilding, sunken cities are often depicted as ancient settlements, but modern cities in post-apocalyptic worlds may be sunken as well.

When designing sunken cities, consider how this settlement came to be submerged. It may have become this way due to a geological event such as a volcanic eruption or tsunami, or may have become submerged because of a magical event. In fictional worldbuilding, sunken cities may contain people or creatures that still live in them. Consider if there were survivors from the original city living in air trapped in the city or an underground, air filled cavern chamber. The city-dwellers may have been prepared for the catastrophe, and the city itself may have been prepared for submersion (in which case it may more resemble an underwater city). The people may have adapted over the years to life underwater. Or, the original inhabitants may be long gone, and the city has been reclaimed by an aquatic species or made home by a leviathan.

Sunken cities may contain valuable resources, and in post-apocalyptic water worlds divers may brave the depths to search for usable materials in the submerged cities below.