Classical Geothermal energy is widely regarded as describing the process of using hot water or steam extracted from geothermal reservoirs in the earth's crust, created by upward convection and conduction of heat from the Earth’s mantle and core, that can be used for geothermal heat pumps, water heating, or electricity generation.

Hot Dry Rock (HDR) describes the process of extracting heat from deep crystalline rock heated by the decay of radioactive elements including uranium, thorium, and potassium isotopes in effect - nature's nuclear power plant.