Traditional geothermal plants, which have existed for decades, work by tapping natural hot water reservoirs underground to power turbines that can generate electricity 24 hours a day. Few sites have the right conditions for this, however, so geothermal only produces 0.4 percent of America’s electricity currently.

But hot, dry rocks lie below the surface everywhere on the planet. And by using advanced drilling techniques developed by the oil and gas industry, some experts think it’s possible to tap that larger store of heat and create geothermal energy almost anywhere. The potential is enormous: The Energy Department estimates there’s enough energy in those rocks to power the entire country five times over and has launched a major push to develop technologies to harvest that heat.

Dozens of geothermal companies have emerged with ideas.

Fervo is using fracking techniques — similar to those used for oil and gas — to crack open dry, hot rock and inject water into the fractures, creating artificial geothermal reservoirs. Eavor, a Canadian start-up, is building large underground radiators with drilling methods pioneered in Alberta’s oil sands. Others dream of using plasma or energy waves to drill even deeper and tap “superhot” temperatures that could cleanly power thousands of coal-fired power plants by substituting steam for coal.

Still, obstacles to geothermal expansion loom. Investors are wary of the cost and risks of novel geothermal projects. Some worry about water use or earthquakes from drilling. Permitting is difficult. And geothermal gets less federal support than other technologies.

Still, the growing interest in geothermal is driven by the fact that the United States has gotten extraordinarily good at drilling since the 2000s. Innovations like horizontal drilling and magnetic sensing have pushed oil and gas production to record highs, much to the dismay of environmentalists. But these innovations can be adapted for geothermal, where drilling can make up half the cost of projects.

“Everyone knows about cost declines for wind and solar,” said Cindy Taff, who worked at Shell for 36 years before joining Sage Geosystems, a geothermal start-up in Houston. “But we also saw steep cost declines for oil and gas drilling during the shale revolution. If we can bring that to geothermal, the growth could be huge.”

tates like California are increasingly desperate for clean energy sources that can run at all hours. While wind and solar power are growing fast, they rely on fossil fuels like natural gas for backup when the sun sets and wind fades. Finding a replacement for gas is an acute climate challenge, and geothermal is one of the few plausible options.

“Geothermal has historically been overlooked,” Senator Lisa Murkowski, Republican of Alaska, said at a hearing. But with innovation, she added, “the potential is out there, I think, that’s pretty extraordinary.”

Fracking for clean energy

Near the town of Milford, Utah, sits the Blundell geothermal plant, surrounded by boiling mud pits, hissing steam vents and the skeletal ruins of a hot springs resort. Built in 1984, the 38-megawatt plant produces enough electricity for about 31,000 homes.

The Blundell plant relies on ancient volcanism and quirks of geology: Just below the surface are hot, naturally porous rocks that allow groundwater to percolate and heat up enough to create steam for generating electricity. But such conditions are rare. In much of the region, the underground hot rock is hard granite, and water can’t flow easily.

Three miles east, two teams are trying to tap that hot granite. One is Utah FORGE, a $220 million research effort funded by the Energy Department. The other is Fervo, a Houston-based start-up.

Both use similar methods: First, drill two wells shaped like giant L’s, extending thousands of feet down into hot granite before curving and extending thousands of feet horizontally. Then, use fracking, which involves controlled explosives and high-pressure fluids, to create a series of cracks between the two wells. Finally, inject water into one well, where it will hopefully migrate through the cracks, heat up past 300 degrees Fahrenheit and come out the other well.

This is “enhanced geothermal,” and people have struggled with the engineering difficulties since the 1970s.