A cistern is a large container or tank built to collect and store water, especially rainwater. Think of it as a giant water savings account: instead of letting rain run off your roof and disappear into the ground, a cistern catches it and keeps it for later use.

For thousands of years, people in dry climates built cisterns to survive. Ancient Romans constructed enormous underground cisterns to supply their cities with water. In places where rain doesn't fall regularly, a good cistern might mean the difference between having water during the dry season and going without. Some cisterns are built underground to keep the water cool and clean, while others stand above ground as large tanks.

You might see cisterns on farms today, collecting rainwater for animals to drink or for irrigating crops. In some parts of the world, families still depend on cisterns as their main source of water. Even in modern cities, some buildings use cisterns to collect rainwater for watering gardens or flushing toilets, saving treated drinking water for more important needs.

A cistern is different from a well, which reaches down to underground water sources, or a water tower, which stores and distributes water under pressure.