Geothermal means earth-heat. It is related to the
thermal energy of Earth’s interior. On a large scale,
the intensity of this thermal energy increases with
depth, that is, the temperature of the Earth increases
as we travel closer to its center. A global average for
Earth’s geothermal gradient (temperature increase with
depth) is approximately 30°C/km. For example, if we
merely removed the outer 3 km of Earth’s outer surface,
it would be a sphere 5000°C at the core, and nearly hot
enough to boil water on its surface. Earth contains an
incredibly vast amount of thermal energy.