Geothermal energy holds tremendous promise in many parts of the world. It uses the heat of the earth (underground "volcanoes") to create energy
Underground (usually over 10km deep), the earth is quite hot (500+°C). Sometimes, at hot springs, this heat wells up to the surface. At these points, water can be turned to steam. With a turbine and generator, we can harness the power created by this rapid change in state. Companies like Calpine and Cathness are turning these heat resources into big money.
Geothermal heat pumps
Although they may not be as thrilling as geothermal turbines, heat pumps are actually much more practical methods for capturing geothermal energy and using it close to the point of capture. Basically, most anywhere you go, the ground is about 55 degrees Farenheit. That "heat" can be used to keep buildings at a constant temperature. So, heat pumps reduce heating bills in the winter and cooling bills in the summer.