
There's energy in them rocks!
Thermal (heat) energy in deep rock can be used to generate steam that in turn can be used to generate electricity.
>>> And best of all it is virtually unlimited .... as long as the earth has a molten core.
Currently GEOTHERMAL is the 3rd largerst source of renewable energy in the United States
"There are three geothermal power plant technologies being used to convert hydrothermal fluids to electricity. ...dry steam, flash and binary cycle. The type of conversion used depends on the state of the fluid (whether steam or water) and its temperature. "

"The Earth's magma and hot dry rock will provide cheap, clean, and almost unlimited energy as soon as we develop the technology to use them. In the meantime, because they're so abundant, moderate-temperature sites running binary-cycle power plants will be the most common electricity producers."
"Before geothermal electricity can be considered a key element of the U.S. energy infrastructure, it must become cost-competitive with traditional forms of energy. The U.S. Department of Energy is working with the geothermal industry to achieve $0.03 to $0.05 per kilowatt-hour. We believe the result will be about 15,000 megawatts of new capacity within the next decade."
Via: US Department of Energy, Geothermal Technologies Program Link