Cisterns are large, artificial, underground reservoirs of water or other liquids. They are often built to catch and hold rainwater and are often present in areas where water may be scarce, such as deserts or badlands. Cisterns differ from 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.