Miles below the Earth’s surface, there’s a source of renewable energy that could sustain all of humanity for the foreseeable future.

Just 0.1% of Earth’s total heat content could meet our energy needs for 2 million years, according to ARPA-E, the government agency that funds R&D efforts for advanced energy technologies.

It’s called geothermal energy, and in some ways, it’s old news.

It’s been used to heat buildings since the late 1800s and provide electricity since the 1900s.

The U.S. has the most installed geothermal capacity in the world, but it still only accounts for about 0.4% of our total electricity mix.

That’s because, in most places, it’s too expensive and challenging to drill geothermal wells.

Over the last few years, a number of start-ups in the geothermal space have gained traction, such as Eavor Technologies, Fervo Energy, Sage Geoystems, and GreenFire Energy.