Geothermal power

Thermal or electrical power produced from the thermal energy contained in the Earth geothermal energy. Use of geothermal energy is based thermodynamically on the temperature difference between a mass of subsurface rock and water and a mass of water or air at the Earth's surface. This temperature difference allows production of thermal energy that can be either used directly or converted to mechanical or electrical energy.

Link:http://en.wikipedia.org/wiki/Geothermal_energy