The earth is hot, and that heat can be used. Even at depths humans can reach, the temperature is several thousands of degrees Celsius. An enormous amount of energy is stored in heat. This source of energy is permanently available, making it a useful addition to sources that are not available continually, such as solar and wind energy.

See the example of Geothermal Heat in The Hague.

Scheme of how energy is stored in the ground and can be transported from and to the nearby buildings © Aardwarmte Den Haag