On the Earth’s surface, temperatures can swing from blistering hot to freezing cold. But dig a few hundred feet below ground, and the temperature is stable at between 10°C and 16°C (50°F and 60°F). The technology to tap that energy to heat or cool homes has been around for 70 years, but it’s only in the past few years that it has become feasible financially to deploy geothermal heat pumps at scale for ordinary consumers.