Power obtained by using heat from the Earth's interior. Most geothermal resources are in regions of active volcanism. Hot springs, geysers, pools of boiling mud, and fumaroles are the most easily exploited sources. The ancient Romans used hot springs to heat baths and homes, and similar uses are still found in Iceland, Turkey, and Japan. Geothermal energy's greatest potential lies in the generation of electricity. It was first used to produce electric power in Italy in 1904. Today geothermal power plants are in operation in New Zealand, Japan, Iceland, Mexico, the U.S., and elsewhere.

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• Geothermal (geology), heat that comes from within the Earth
• Direct exchange geothermal heat pump, a method of heating and cooling with the energy of the earth using direct exchange of heat
• Geothermal desalination, the production of fresh water using heat energy extracted from underground rocks
• Geothermal heating, a method of heating and cooling a building using underground heat
• Geothermal power, electricity generated from naturally occurring geological heat sources
• Hot dry rock geothermal energy, Heating water in hot deep rock
• Geothermal heat pump, a method of heating and cooling using a heat pump with the earth as a heat reservoir
• Earth sheltering, constructing a building into a hill side or Earth berm to reduce heating and cooling requirements
• Earth cooling tubes, using ambient Earth temperature to cool and dehumidify air