Jules Verne knew it — the center of the Earth is packed with heat energy. Humans have been using underground thermal springs as a source of hot water for centuries — just visit an onsen resort in Japan. But geothermal energy can also be used to make electricity, by taking the ultra-hot water found beneath certain parts of the Earth's surface, converting it to steam and then using the steam to drive an electric turbine. Right now geothermal energy is mostly used in countries with unusually active volcanic surfaces, like Iceland, where more than a quarter of the country's electricity comes from geothermal sources. But you don't need volcanic eruptions to make use of geothermal energy. Geothermal heat pumps can generate carbon-free heating and cooling nearly anywhere in the world. And the U.S. has significant geothermal potential for electricity as well: the government estimates that geothermal could provide up to 10% of the country's electrical supply.