A river of hot water flows some 3,000 feet beneath Boise, Idaho. And since 1983 the city has been using that water to directly heat homes, businesses, and institutions, including the four floors of city hall — all told, about a third of the downtown. It’s the largest geothermal heating system in the country.

Boise didn’t need to drill to access the resource. The 177-degree Fahrenheit water rises to the surface in a geological fault in the foothills outside of town.

It’s a renewable energy dream. Heating the 6 million square feet in the geothermally warmed buildings costs about $1,000 a month for the electricity to pump it. (The total annual cost for depreciation, maintenance, personnel, and repair of the city’s district heating system is about $750,000.)

“We’re heating 92 of the biggest buildings in the city of Boise,” said Jon Gunnarson, the city’s geothermal coordinator. “The buildings strip heat, collect it, and run it to an injection well. We use it once and reinject it and use it again.”

The Boise district system is how geothermal energy is most often thought of — natural hot water is pumped into radiators or used to generate electricity. It is considered a local phenomenon — few places are sitting on an underground river of steaming hot water — and so geothermal has not been viewed as a major feature on the alternative energy landscape.

But a number of experts around the world say that notion is wrong. Thanks especially to the deep-drilling techniques and knowledge about underground formations developed by the oil and gas industry during the fracking boom, a type of geothermal energy called deep geothermal can access hot temperatures in the earth’s mantle as far down as two to three miles. At various depths up to this level, much of the planet contains extremely hot water or there is hot rock into which water can be injected and heated, a technology known as enhanced geothermal systems. In either case, the hot water is pumped out and used to directly heat buildings or to generate electricity with steam or hot water.

“Wherever we are on the surface of the planet, and certainly the continental U.S., if we drill deep enough we can get to high enough temperatures that would work like the Boise system,” said Jefferson Tester, a professor of sustainable energy systems at Cornell University and a leading expert on geothermal energy. “It’s not a question of whether it’s there — it is and it’s significant. It’s a question of getting it out of the ground economically.”

Even though geothermal is barely on the alternative energy radar, the U.S. already produces 3.7 gigawatts (GW) of geothermal electricity, enough to power more than 1 million homes. It’s the world’s leading producer — primarily in central California and western Nevada. California has 43 operating geothermal generating plants, and is about to build two more.

With mandated renewable energy targets in many locales and the Net Zero campaign — a commitment by many countries to decarbonize their economies by 2050 — interest in geothermal energy is growing rapidly. Many experts see it as an essential component of the world’s green-energy future because it could provide carbon-free heat and around-the-clock baseload power to compensate for the intermittency of wind and solar. Iceland, which sits on an active geological fault line, perfected the technology with its ubiquitous geothermal district heating systems.

And China is embracing the move to geothermal power, currently developing more geothermal district heating systems than any other nation.

“Geothermal electricity is always on ,” said Tester. “It can provide fully dispatchable power or heat and is scalable in the same way other renewables are.”

In a recent report, the International Renewable Energy Agency (IRENA) predicted the output of geothermal in Europe could increase eight-fold by 2050. And a 2019 U.S. Department of Energy (DOE) report — GeoVision: Harnessing the Heat Beneath Our Feet — refers to the “enormous untapped potential for geothermal.” By overcoming technical and financial barriers, the report says, generating electricity through geothermal methods could increase 26-fold by 2050, providing 8.5% of the United States’ electricity, as well as direct heat.

“GeoVision models indicate the opportunity for more than 17,500 direct-heating installations as well as heating and cooling for the equivalent of more than 28 million households using geothermal heat pumps by 2050,” wrote Susan Hamm, director of DOE’s Geothermal Technologies office.

Accessing deep geothermal is expensive and risky. But with a global focus on decarbonizing economies, many countries could offer risk-mitigation strategies and financial incentives, including tax benefits, cost sharing, and technical research. The U.S. Department of Energy, for example, has committed $150 million to the Frontier Observatory for Research in Geothermal Energy (FORGE) project in Utah as a study and test site for enhanced geothermal.

Experts expect geothermal will receive a boost from the incoming Biden administration, which — in stark contrast to the Trump administration — has made a major commitment to slashing the U.S.’s greenhouse gas emissions

Several types of geothermal are increasingly being deployed these days. One, which has been around for decades, has nothing to do with hot water. It’s called “ground source heat pump technology” (GSHP), and it capitalizes on the difference in air temperature and ground temperature. In winter in cold climes, the ground, at 55 degrees F or so, is warmer than outdoor temperatures — and pipes with antifreeze circulate through the ground and pump that warm fluid into a home. Supplemental heat can bring up the temperature. In summer, the ground stays cooler and this same system can provide air conditioning. Sweden is a world leader in this approach and a fifth of the homes there use GSHP. A major hurdle for many homeowners, however, is the cost, which can run into the tens of thousands of dollars per dwelling.

Another very different kind of geothermal is a deep closed loop. A system called the Eavor Loop is operating in Alberta, Canada. It features a single hole drilled down about 1.5 miles, then laterally for a similar length through extremely hot geological formations, and then back up to the surface. The entire hole is lined with pipe and then filled with fluid. It’s a kind of natural radiator. As the heat-transfer solution travels through the length of pipe, it’s heated by the high ground temperatures and then flows to the surface.