Cisterns (synonyms: wells, tanks; example toponyms: Central Cistern) are large, artificial, underground reservoirs of water or other liquids. They are often built to catch and hold rainwater and are particularly present in areas where water may be scarce, such as deserts. Cisterns are different than wells as cisterns must be actively filled and act as a storage area, whereas wells tap into existing groundwater and are replenished via that groundwater source. Cisterns may be full or empty and may exist as an underground ruin in fictional worlds. They may lead to underground cave systems or the sewers.

When designing cisterns for your fictional world, consider their size as well as their need. Cisterns are not always filled with water, and may be used for some other important liquid resource in your world.