Almost everywhere, the ground 10 feet (3m) below the Earth's surface maintains a nearly constant temperature between 50° and 60 °F (10° and 16 °C). Geothermal heat pumps can tap into this resource to heat and cool buildings. A geothermal heat pump system consists of a heat pump, an air delivery system (ductwork), and a heat exchanger - a system of pipes buried in the shallow ground near the building. In the winter, the heat pump uses power to remove heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump uses power again to move heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to provide a free source of hot water. The power to run the heat pump comes from another source.