Geothermal energy is obtained from hot rocks beneath the earth's surface. The planet's core, which may generate temperatures as high as 8,000°F (4,500°C), heats its interior, whose temperature increases, on an average, by about 1°C (2°F) for every 60 ft (18 m) nearer the core. Some heat is also generated in the mantle and crust as a result of the radioactive decay of uranium and other elements.

In some parts of the earth, rocks in excess of 212°F (100°C) are found only a few miles beneath the surface. Water that comes into contact with the rock will be heated above its boiling point. Under some conditions, the water becomes super-heated, that is, is prevented from boiling even though its temperature is greater than 212°F (100°C). Regions of this kind are known as wet steam fields. In other situations the water is able to boil normally, producing steam. These...