Geothermal energy is the energy produced by the planet itself. Heat energy is constantly being produced in the Earth's core via radioactive decay. This intense heat (5000 degrees Celsius) is radiated outward from the core toward the surface of the planet. We can tap into this energy to use it either for the production of electricity or for direct heating of buildings. Boring down deep into the Earth's crust, the heat of the Earth can be piped to the surface in the form of steam or heated water to drive a turbine and produce electricity or directly into an individual heat pump for home use. Geothermal energy is considered a sustainable source of energy because the energy extracted from the planet is only a tiny fraction of the total energy naturally produced by the planet. Currently, only a tiny fraction of the world's energy is produced via geothermal means. Iceland probably produces more geothermal energy as a percent of total energy due to its small size/population and its proximity to tectonic activity.