Endurance Energy Tests the World's First Subsea Geothermal Well: What the Axial Seamount Demonstration Really Proved What if geothermal energy did not need a power plant on land? What if electricity could be produced directly from the Earth's heat beneath the ocean, with generators sitting on the seafloor and sending power to coastal grids? On October 19, 2026, Seattle-based startup Endurance Energy announced a major step toward that idea. The company said it had drilled and completed the world's first subsea geothermal well and placed a 3-ton test power plant nearly a mile beneath the Pacific. It also reported net-positive electricity from heat at an active underwater volcano, according to its announcement on PR Newswire. The headline numbers are modest. The Adélie prototype produced more electricity than it used for about 8.7 seconds. During that time it delivered a median net output of about 2 kilowatts and a peak of 16.5 kilowatts. One detail matters more than any other...
Endurance Energy Tests the World's First Subsea Geothermal Well: What the Axial Seamount Demonstration Really Proved
What if geothermal energy did not need a power plant on land? What if electricity could be produced directly from the Earth's heat beneath the ocean, with generators sitting on the seafloor and sending power to coastal grids?
On October 19, 2026, Seattle-based startup Endurance Energy announced a major step toward that idea. The company said it had drilled and completed the world's first subsea geothermal well and placed a 3-ton test power plant nearly a mile beneath the Pacific. It also reported net-positive electricity from heat at an active underwater volcano, according to its announcement on PR Newswire.
The headline numbers are modest. The Adélie prototype produced more electricity than it used for about 8.7 seconds. During that time it delivered a median net output of about 2 kilowatts and a peak of 16.5 kilowatts.
One detail matters more than any other. The newly drilled well produced hot fluid but could not supply enough flow to run the plant. The net-positive run came after the team connected Adélie to a nearby natural hydrothermal vent, as both Ars Technica and Canary Media reported.
So the test has not yet shown that a drilled subsea well can keep a power plant running. What it did show is that a bold concept has moved from theory into real engineering. For the geothermal industry, that is worth watching closely.
Inside the Axial Seamount Demonstration
Axial Seamount is an active submarine volcano on the Juan de Fuca Ridge, about 300 miles west of Oregon. Seawater seeps into cracks in the oceanic crust there, gets heated by hot rock below, and comes back out through hydrothermal vents. At deep-ocean pressures, that fluid can be far hotter than the normal boiling point of water.
The site was chosen with care. Heatmap describes Axial Seamount as the only place in the world where a cluster of hydrothermal vents is connected to shore by fibre-optic cable. That cable let Endurance watch and control its equipment in real time from Seattle, according to Heatmap.
The expedition sailed in September aboard the vessel Ocean Guardian and spent three weeks at the site, according to Canary Media. The campaign brought together several operations that had never been combined on the seafloor:
Drilling into the seabed and hitting hydrothermal fluid.
Casing, cementing and completing a geothermal well.
Fitting a valve to control the well.
Landing a power plant weighing about 3 tons.
Testing heat transfer and turbine operation.
Controlling the equipment remotely from shore.
Measuring electricity produced against the power the system used.
Each of these steps has parallels in offshore oil and gas or onshore geothermal. Doing all of them together, on an active volcano, about 1,500 metres down, is a very different challenge. The equipment has to handle crushing outside pressure, corrosive seawater and very hot fluids, with almost no chance of hands-on repair.
Adélie was Endurance's fifth deep-sea deployment, but the first to combine drilling, power generation and data links to Seattle in one mission, Canary Media noted. It was more than a turbine test. It was a test of a whole offshore geothermal operating model.
The First Subsea Geothermal Well: What Was Actually Drilled
Endurance's subsea lander drill bored two holes in the Axial Caldera, less than two feet apart, about 5,000 feet below the ocean surface. Both struck hydrothermal fluid. The team then cased and cemented the second hole, capped it with a valve, and completed a well 16 metres (52 feet) deep. After cementing, it flowed at about 260°C (500°F), according to the company.
On land, developers often drill one to three kilometres to reach fluids that hot. Reaching 260°C at 16 metres shows why the seafloor is so appealing. But temperature alone does not make a commercial reservoir. A geothermal project also needs enough flow, suitable pressure, steady production over time, scaling it can manage, and equipment that keeps working for years.
The well was deliberately small, about two inches wide. Oil, gas and onshore geothermal wells often start with casings 20 to 36 inches across. The first goal was to prove that a well could be drilled and completed on the seafloor, not to maximise production. The narrow borehole limited how much fluid could reach the plant.
Endurance says its current lander drill can reach about 80 metres (260 feet), five times the depth of the first well. Deeper wells may reach more hydrothermal fluid and deliver higher flow. Whether that flow will be enough for sustained generation still has to be proven.
The lesson will be familiar to any geothermal developer. Drilling a well is a necessary milestone, but reservoir productivity decides whether a resource becomes a power project.
How the Adélie Power Plant Works
Adélie uses an organic Rankine cycle, the same basic technology used at many land-based geothermal plants, according to Canary Media. Heat from the hydrothermal fluid passes through a heat exchanger into a separate working fluid. That fluid vaporises, spins a turbine connected to a generator, and is then cooled and condensed so the cycle can repeat.
The deep ocean gives this design two natural advantages. "The deep ocean gives us unlimited cold seawater for cooling, and its weight keeps the system and the well pressurized, so our plants can be compact," Andrew Redd, Endurance's founder and CEO, told Ars Technica.
Still, a hot resource does not automatically produce a lot of electricity. Net output depends on several factors working together:
The temperature and flow rate of the geothermal fluid.
How efficient the heat exchanger and turbine are.
How much power the pumps, controls and other auxiliary equipment use.
Heat lost across the system.
How well the cooling and working-fluid loops perform.
How reliable the electrical connections and protection systems are.
A plant that makes electricity but uses almost as much to run its pumps and controls delivers little useful power. That is why the net-positive result matters.
The 8.7-Second Breakthrough in Numbers
On September 27, 2026, Endurance connected Adélie to a natural vent with fluid at about 281°C (538°F). The turbine reached about 26,000 rpm. The run lasted about 29 seconds in total and produced about 60 watt-hours of electricity, according to Canary Media. For nearly 9 of those seconds the plant made more power than it used, and the surplus went into charging its backup battery
These figures need careful reading. A kilowatt measures power, the rate at which electricity is produced. A kilowatt-hour measures energy, the amount produced over time. The 16.5-kilowatt peak was a brief high point, not a sustained output. The 60 watt-hours from the run is roughly what a laptop uses in an hour or so. It should not be mistaken for 60 kilowatt-hours or for continuous production.
What matters is that the system crossed the line from using more electricity than it made to producing a net surplus. A self-sustaining subsea power plant has to clear that hurdle first.
Redd compared the moment to an early milestone in rocketry. "Goddard's first liquid-fueled rocket flew for two and a half seconds. Our first net-positive run lasted about nine. Our approach is to make it exist, then make it good," he said in the company announcement.
It also helps to compare the result with the original plan. Before the deployment, Adélie was described as a 100-kilowatt system meant to run at Axial Seamount for a year, connected to the Ocean Observatories Initiative's cabled network, according to NationalWorks. The September result fell well short of that ambition. The test established a technical possibility. Turning it into a reliable energy asset is the next challenge.
Why the First Well Did Not Power the Plant
This is the most important caveat in the whole announcement. The completed well reached hot fluid at about 260°C, but the narrow borehole was too restrictive to deliver enough heat for the generator to run, Heatmap reported. The team then connected Adélie to a nearby, hotter natural vent, and that is where the net-positive run happened.
Two separate achievements should not be confused:
Drilling and completing a subsea geothermal well, which showed that seafloor well construction is possible.
Producing net-positive electricity from a natural hydrothermal vent, which showed that the power system can briefly make more electricity than it uses.
Neither result on its own proves that a purpose-built subsea well can keep a power plant running. Future deployments have to join these two capabilities into one sustained system. The well must deliver enough fluid, the plant must convert that heat efficiently, and the whole installation must stay running long enough to produce useful amounts of electricity.
Battery Protection and Cable Failure Show the Engineering Challenge
The net-positive run did not last. Adélie relies on battery power to run its pumps, thrusters and controls and to bring the turbine up to speed. During the test, a battery module showed a brief current spike and stopped reporting, and the plant's software shut the system down as a safety measure, according to Ars Technica.
About ten minutes later, the team tried to restart. A power cable feeding the refrigerant pump failed and the dive ended. Ars Technica reported that a defect in the cable's outer casing let seawater in and shorted two conductors.
These failures show why a seafloor power plant takes far more than putting a turbine underwater. High-temperature fluid handling, power electronics, batteries, pumps, communications, control software and electrical protection all have to work together. One failed component can stop the whole operation.
On land, a technician can often swap a faulty part in hours. At about 1,500 metres down, the same repair can require a support vessel, a remotely operated vehicle, special procedures and a lot of money. Subsea geothermal developers have to design for reliability from the start, with redundancy, remote diagnostics and planned ways to replace components.
Endurance has been open about these problems. The company says the deployment produced data on seafloor drilling, well flow, heat transfer, mineral buildup, high-temperature generation and long-distance control of subsea equipment, according to its release. That knowledge may prove more valuable than the electricity produced.
A Lighter Drilling Footprint Than Offshore Oil and Gas
Offshore drilling has a poor environmental reputation, mostly because of oil and gas. Endurance appears to have designed its approach to look very different. It drilled away from vent chimneys and the animals that live around them. Its well was only about two inches wide. It also used filtered seawater instead of drilling mud, avoiding the chemicals common in offshore oil and gas work, according to the company.
This slim, mud-free approach is only possible because the heat is so shallow. That is a structural advantage of subsea geothermal. The trade-off, as Adélie showed, is flow. Endurance will need deeper wells, or more of them, while keeping its footprint small.
Environmental Monitoring at Hydrothermal Vents
The ocean floor is not an empty industrial site. Hydrothermal vents support unusual ecosystems of tubeworms, snails, crabs and microbial mats adapted to extreme heat and chemistry. Any commercial activity there will face close scrutiny.
Endurance ran a voluntary environmental monitoring programme throughout the deployment. Before placing equipment, the team mapped the seafloor and chose the least disruptive site. It sampled water before and after every operation and filmed the surrounding seafloor life on every dive, according to the company.
The team checked its findings against 11 instruments run by outside scientists. The loudest sound recorded was the ship's engine, which was within the normal range for research vessels at Axial Seamount. No marine mammals were seen. Monitoring found no unusual seismic or volcanic activity, no significant change to the seafloor community and no significant change to the site's natural hydrothermal activity. More than 40 water, rock and mineral samples are now with independent laboratories.
These are early results from a short, small-scale test, and they are mostly self-reported. The independent lab findings will matter. Still, building an environmental baseline from the first deployment is a smart move. Offshore wind learned that public acceptance can delay projects as much as technology or finance can. A subsea geothermal industry that arrives with credible data will be in a far stronger position when it applies for permits.
Why Subsea Geothermal Could Matter to Pacific Island Nations
Endurance is building its technology with island nations and coastal grids in mind. Its first commercial project is in development in the Kingdom of Tonga, targeting first power in 2027 and aiming to feed clean, always-on electricity into a grid that now runs mostly on imported diesel, according to the company.
Tonga's need is clear. Diesel supplied about 83% of the country's electricity in 2023, and renewables supplied 16.66%, according to Tonga's review report to the UNFCCC. The same report says Tonga's target of 70% renewable electricity by 2030 is not on track. The main grids burned about 18 million litres of diesel in fiscal year 2022–2023, leaving the country highly exposed to oil price shocks, according to the Asian Development Bank.
Solar and batteries have helped, but small island grids struggle to absorb large amounts of variable power without costly storage. A firm, round-the-clock source like geothermal could fill exactly that gap. Endurance is developing Leopard, a 2-megawatt commercial generator, and several of these units would make a real difference on a grid of Tonga's size.
The Tonga project is still a development target, not an operating plant. Its economics will depend on resource quality, installation costs, equipment reliability, subsea transmission, environmental approvals and the cost of the diesel and renewables it would compete with. The real question for island nations is not whether subsea geothermal can make electricity. It is whether it can deliver that electricity reliably and at a price that beats what they have now.
A $63 Million Bet on Subsea Heat
Endurance has raised $63 million so far, including a $54 million Series A led by Founders Fund, according to its announcement. That money gives the company room to refine its prototype, drill deeper wells and build larger systems.
The funding also shows that investor appetite for next-generation geothermal now reaches beyond enhanced geothermal and superhot rock projects on land. Data centres, industrial sites and utilities increasingly want clean power that is available all the time, not just when the sun shines or the wind blows. Subsea geothermal offers one more possible source.
But investment is not proof of commercial viability. Investors will want evidence of reliable operation, productive wells, maintenance costs they can manage and a credible path to project-level returns. Until operating data exists, the financial case rests on milestones that have not yet been reached.
What Comes Next: Gentoo, Deeper Wells and Leopard
The roadmap is clear. This winter, Endurance plans to deploy Gentoo, a refurbished and upgraded version of Adélie aimed at higher power and longer runs, according to World Oil. The company expects wells closer to its drill's 80-metre limit to deliver the flow needed for sustained generation. At the same time, it is developing the 2-megawatt Leopard generator for commercial use.
Going from a 16.5-kilowatt peak to a 2-megawatt generator is roughly a 120-fold jump. The modular design helps. Instead of one giant offshore plant, standard units could be installed in clusters across a hydrothermal field, much as small wellhead generators have brought early power online at onshore geothermal fields.
Key Risks the Industry Should Watch
Sustained flow: can deeper wells supply enough fluid for megawatt-scale output, and how fast will flow decline?
Scaling and corrosion: vent fluids are full of dissolved minerals that can clog heat exchangers and wells over months and years.
Power export: Adélie used an existing scientific fibre network, but commercial projects will need subsea power cables to shore.
Maintenance and uptime: deep-sea repairs are slow and expensive, so reliability will drive the cost of electricity.
Regulation: most countries have no permitting framework for subsea geothermal or for activity near vent ecosystems.
Volcanic hazard: building on active volcanoes adds geological risk that insurers and lenders will have to price.
These are normal hurdles for any first-of-its-kind energy technology. They also explain why the step from nine seconds of net power to years of bankable generation is the real test.
Conclusion: Nine Seconds That Opened a New Geothermal Frontier
Endurance Energy's Adélie test did not power a town. It ran net-positive for 8.7 seconds, produced about 60 watt-hours, and ended with a failed cable. Its drilled well also could not run the plant. But the company did drill the world's first subsea geothermal well, reach 260°C fluid just 16 metres below the seabed, and control a power plant about 5,000 feet down from Seattle. It also showed that a seafloor geothermal system can make more power than it uses.
The next milestones are clear: longer, stronger runs with Gentoo this winter, deeper wells, the 2-megawatt Leopard generator and first commercial power for Tonga's diesel-dependent grid in 2027. If Endurance hits those targets, the ocean floor could become geothermal energy's next great frontier. In Redd's words, the goal is to "make it exist, then make it good." Subsea geothermal power now exists. The industry will be watching to see how good it can get.
Related: Endurance Energy raises $54M to harness a massive untapped energy source on the ocean floor
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source: Finance Yahoo
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