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By “other idea,” Redfern is referring not just to geothermal’s place within the pantheon of renewables, but also to differing approaches within his own industry. There are two major schools of thought. One category includes efforts like Eavor’s, known as “closed-loop” or “advanced” geothermal systems (AGS). Their goal is to drill a circuitous borehole that is filled with water and conducts heat to the surface. “We’re hermetically sealed,” Redfern explains. “We have no water going in or out of our system into the rock.” The challenge is in transferring enough heat from the rock, which will cool in turn, until its heat is replenished from the fiery depths. To make a sustainable system, you need a lot of surface area and thus, to lay a lot of pipe.

In the other category, enhanced geothermal systems (EGS) rely on two deep parallel holes that are drilled out then “fracked” with a high-pressure blast of water. This cracks open fissures in the rock between the two holes. Engineers are often fans of this approach because it’s an efficient way to create lots of surface area for water to come into contact with hot rock—so much area that the water can be pumped through the system much faster without cooling the surrounding rocks too much. Unlike fracking for oil and gas, the water is typically recycled after each use. Historically, though, EGS has been hampered by the challenges of precisely engineering those fractures. Some projects failed when water disappeared into hidden faults and never returned to the surface, others when the fracking produced damaging earthquakes.

“There’s this big debate, and sometimes it gets pretty intense,” says Koenraad Beckers, a geothermal researcher at the National Renewable Energy Laboratory. “Some people say AGS is the holy grail but others say it’s EGS.” Recently, he adds, EGS has taken a clear lead, especially in the U.S., where it is the darling of both government research efforts and investors, with startups like Fervo building out large-scale systems in the American West. Because they don’t require drilling such vast distances, EGS also has the benefit of being more cost-efficient to drill in very hot rock, where drilling even a small distance is especially challenging and expensive. That’s a key advantage for producing power, because the hotter the water is, the easier it can be converted into electricity.

Hence closed-loop AGS as the “other idea,” in more senses than one. Roland Horne, a professor of earth science at Stanford University, has watched people attempt closed-loop ideas for about a half-century. He was recently part of a DOE-backed analysis that found that making all the extra pipe required of deep closed-loop systems worth it would require drilling costs to drop by half. Consider him skeptical. “If they do it, it would be a tremendous advance,” he says.

Then again, drilling costs are falling more rapidly than anyone in the industry imagined. In 2022, the DOE set a goal of shaving 90% off the cost of EGS technology by 2035, which seemed ambitious at the time. Fervo recently reported that costs were falling faster than required to reach the agency’s ambitions. “If you’d asked me two years ago if it would be possible, I would have said no,” Horne says of closed-loop efforts. Nowadays—maybe.

Advocates like Redfern maintain that if they can make the drilling work, Eavor’s approach will tap heat with more control than fracking, which requires careful calibration to each new geology—and, despite improvements in techniques, is still dogged by the specter of earthquakes. In the long term, he says, a more repeatable design like his will be key to scaling up AGS. Plus, there are plenty of places where fracking simply isn’t advisable due to local geology, or where public perception—soured by the legacy of fracking for gas or by geothermal’s earthquake issues—doesn’t welcome it.

For the past few years, Eavor has been working to show that they can be fast. First was a test site in Alberta, Eavor’s home state, designed to connect two wells in a single loop. Next it moved to a site in New Mexico to test drilling deeper—nearly 5,500 meters, where temperatures approach 250°C.

That’s as piping hot as a pizza-ready oven, and the point at which digging tools borrowed from the oil and gas industry start to give out. Eavor demonstrated a cooling system there that sprays water on the drill bits as they heat up.

But Germany offered an opportunity to actually start producing energy. The Geretsried site was meant to be a traditional geothermal project but failed in 2009 after its backers failed to find enough naturally hot water. Germany is particularly eager to try out closed-loop. The country has banned forms of fracking since 2017, and even though the ban primarily targets gas exploration, EGS has its own tarnished legacy in the neighborhood. Just over the Alps, Switzerland was an enthusiastic EGS experimenter until a 3.4 magnitude earthquake induced by fracking shook the ancient church steeples of Basel in 2006. (Smaller earthquakes from the frack have continued ever since.) The first quake largely paralyzed EGS experiments in the region.

So in this case, the “other idea” looked like just “the idea.” Geretsried’s rock was unusually warm by European standards, reaching 160°C at just about 4,500 meters down—turkey-roasting temperatures, but not pizza. That still might not be enough to produce much electricity, but it was coupled with a new strategy.  “We realized that everyone else was focused on electricity, but the main thing is heating, which is as big a market as electricity in Europe,” he says. In 2020, Eavor partnered with the prior geothermal company, taking advantage of its lease and exploratory work.

Redfern insists that Eavor will eventually turn its attention to hotter environments where power is the priority. In Sonoma, California, the company signed an agreement last year to explore ways to expand the power generation at The Geysers, a collection of 18 traditional geothermal plants that together produce more than 600 megawatts of electricity—enough to power every home in San Francisco. It’s the type of place where closed-loop is, once again, appealing: relatively shallow heat, meaning potentially less drilling, and located in a seismically active area where residents have historically objected to EGS. In 2008, in the wake of the Basel earthquake, an EGS project proposed for the region sparked local protests and national media attention. (The project went forward but failed, due to drilling problems unrelated to the technique.)

But for now, Eavor’s focus is on heat. “We’ve got our hands full,” Redfern says. There are dozens of sites in Europe where communities have either built district heating systems or plan them, he adds, and all could potentially benefit from a deep geothermal energy source to feed them. A few months after the bit christening, German Chancellor Olaf Scholz visited the site and declared it a model for Europe as the continent struggles to find its energy footing after its breakup with cheap Russian natural gas.

European officials are placing a substantial bet that the costs of drilling will go down, subsidizing the €370 million Geretsried development with €92 million from the EU Innovation Fund. An additional €130 million in financing comes from a consortium of banks, including the European Investment Bank and a group of Japanese investors who are similarly interested in closed-loop AGS in an earthquake-prone country that lacks space for solar panels and turbines. Redfern says the plant will be paid between €50 and €90 per megawatt-hour for its thermal energy, and €251 for electricity, but the company declined to say whether its cost estimates were on track to break even, with or without subsidies. Whether that happens will likely determine the fate of dozens of other future projects. “My impression is that they really know what they’re doing and they know it’s going to take a lot of drilling,” says Beckers. “It boils down to, ‘How fast can you drill?’”

The blessed bits of Geretsried have been working hard over the past year, drilling more than 10,000 meters—the vertical portion plus a start on the laterals—while power plant construction proceeds on the surface. It will take tens of thousands of additional meters and until at least 2026, when all four loops and the district heating system are scheduled for completion, to know for sure. “If they make it work, great,” Horne says. “I would applaud that with everyone else.” That’s the thing, after all, about making a dent in global emissions. To get there, even critics have to hope all the other ideas will work, too.

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Gregory Barber is a climate and technology reporter based in San Francisco. He was previously a reporter for WIRED and Mother Jones and won the Walter Sullivan Award for Science Journalism from the American Geophysical Union in 2022.