Geothermal district heating. | Shutterstock The world needs clean heat, and geothermal energy has it. The heat stored in the Earth’s crust, known as geothermal energy, is carbon-free and effectively inexhaustible. There’s enough of it to run all of civilization for generations, if it could be cost-effectively tapped. Tapping it turns out to be no small feat, but efforts have ramped up recently due to new urgency by the climate crisis and the search for low-carbon alternatives to fossil fuels.