Geothermal energy is heat energy that originates within Earth itself. The temperature at the core of our planet is 4,200°C (7,592°F), and heat flows outward to the cooler surface, where it can produce dramatic displays such as volcanoes, geysers, and hot springs, or be used to heat buildings, generate electricity, or perform other useful functions. This outward flow of heat is continually being maintained from within by the decay of radioactive elements such as uranium, thorium, and radium, which occur naturally in Earth. Because of its origin in radioactivity, geothermal energy can actually be thought of as being a form of natural nuclear energy.

The U.S. Department of Energy has estimated that the total usable geothermal energy resource in Earth's crust to a depth of 10 kilometers is about 100 million exajoules, which is 300,000 times the world's annual energy consumption. Unfortunately, only a tiny fraction of this...