Geothermal energy doesn’t seem to get a whole lot of attention; according to MIT, the last in-depth study was done in 1979. This energy traditionally comes from hot springs near the Earth’s surface (like in Reykjavik, Iceland). They emit hot steam, which spins turbines to generate electricity. In contrast to coal, a non-renewable fossil fuel that generates half of the U.S. electrical supply, geothermal energy creates very few emissions; and unlike other "alternative" energy forms, geothermal energy generators can run non-stop without relying on sunlight or wind. 
The U.S. Department of Energy-sponsored study focused on an alternate "Enhanced Geothermal System" that would involve drilling holes more than 5,000 ft toward the Earth's mantle, into hot, dry rock. Water would then be sent down the holes to be heated, and then travel through natural fractures to be sucked up other nearby tunnels (The Department of Energy has a good animation). At the surface, the water would either be cooled quickly to make steam (a process called flashing) or looped around another liquid that could be heated into steam at a lower temperature; that steam would power turbines to generate electricity.
The report notes that water would have to be heated in excess of 150-200°C for electricity or 100-150°C for heating homes. Compare those numbers to the map at right, which shows estimated rock temperatures at more than 20,000 feet below ground.
This sounds completely fascinating to me. If you're still confused about how it all works, the Union of Concerned Scientists has a good page explaining these systems.
MIT-led panel backs "heat mining" as key U.S. energy source (Mass. Institute of Technology)
