This guide provides an overview of geothermal energy systems and how they can be used to generate clean, renewable energy. It covers the basics of geothermal energy, including how it works, the different types of systems, and the benefits
From Wikipedia
Geothermal energy is thermal energy extracted from the Earth's crust. It combines energy from the formation of the planet and from radioactive decay. Geothermal energy has been exploited as a source of heat and/or electric power for millennia.