> ## Content Index > Fetch the complete content index at: https://nexi.fund/llms.txt > Use this file to discover other available public pages before exploring further. # Endurance Energy Proves Subsea Geothermal Can Run Net-Positive - URL: https://nexi.fund/endurance-energy-subsea-geothermal-2026/ - Published: 2026-11-02T18:00:10.000Z - Updated: 2026-11-02T18:00:10.000Z - Description: A three-ton plant on an underwater volcano made more power than it used, for 8.7 seconds. The engineering argument is over; the cable bill decides what comes next. - Author: Nexi.fund Labs - Tags: Energy & Climate, #mode-1, #hook-paradox, #track-E, #layout-signal-grid The turbine spun at 26,000 rpm. The vent ran at 281 °C. For 8.7 seconds, the plant on the seafloor made more electricity than it consumed. That is the entire result from Endurance Energy's Adélie test: about 60 watt-hours over a 29-second run, a median of 2 kilowatts net and a peak of 16.5 kilowatts, measured nearly a mile beneath the Pacific. The duration invites ridicule. It is also the first time a geothermal power plant has been landed on the deep seabed, connected to hydrothermal fluid and run net-positive under remote control. Endurance is a Seattle company founded in January 2025\. It has raised $63 million, including a $54 million Series A led by Founders Fund, and it plans to sell its first commercial power in Tonga in 2027\. The pitch is baseload electricity from subsea volcanoes, built faster than nuclear and without the land constraints that slow conventional geothermal. The test moved that pitch from concept to engineering problem. The engineering problem is not small. ## What 8.7 seconds actually bought Adélie is a 100-kilowatt organic Rankine cycle generator: a three-ton package that boils a working fluid with heat from hydrothermal water, then spins a turbine with the vapour. Endurance lowered it to the Axial Seamount, an active volcano on the Juan de Fuca Ridge roughly 300 miles off Oregon, in about 1,500 metres of water. Engineers in Seattle ran the plant over the National Science Foundation's Regional Cabled Array, a seafloor fibre network operated by the University of Washington. A robot connected the unit to a natural vent. The turbine reached operating speed. A battery protection circuit then tripped and cut the run. 16.5 kW peak net #### Peak net output, Adélie Median net output was about 2 kW during the net-positive window. *Endurance Energy, 19 Oct 2026* 8.7 seconds net-positive #### Run length, 27 September Total generation lasted about 29 seconds; the surplus lasted 8.7\. *Endurance Energy, 19 Oct 2026* Ten minutes later, a power cable feeding the refrigerant pump failed during a restart attempt. That ended the dive. The run was short, but it answered the narrow question the company set out to answer. A plant can be landed, connected, controlled from shore and produce net power at depth. The failures were in auxiliary systems, not in the geothermal loop. Endurance recovered, serviced and redeployed the unit in under 16 hours. That number matters as much as the 8.7 seconds. Subsea equipment that cannot be repaired quickly becomes a liability, and the company's fifth deployment in just over a year was its first full rehearsal of the repair cycle. ## The well is the hard part, and the well underdelivered The milestone came from a natural vent, not from the well Endurance drilled. That distinction is the most important technical caveat in the release. The lander drill bored two holes less than two feet apart at about 5,000 feet below the surface. Both hit hydrothermal fluid. The team cased and cemented the second hole, capped it with a valve and completed a 52-foot well. After cementing, it flowed at 260 °C. It did not flow fast enough to run the plant. Endurance moved Adélie to a nearby vent at 281 °C to finish the test. The binding constraint is flow. Temperature is abundant; a 16-metre well in fractured volcanic rock is a straw. The company's current drill can reach 80 metres, and deeper wells should intercept more permeable fractures. Whether they do, at a cost that supports commercial generation, remains unproven. The site also imposes costs that onshore geothermal avoids: saltwater corrosion, high pressure, mineral precipitation inside heat exchangers, and the price of sending a ship and a remotely operated vehicle to fix anything that fails. Endurance calls these lessons, which is the right framing for a test article. One more detail from the release deserves attention. The well was completed with a valve and flowed on its own. A subsea geothermal field would need several such wells feeding one plant, or one very good well, to run continuously. The drilling record so far is two holes, both live, one unusable for the plant. That is a small sample. ## Tonga is the real test case Endurance has a memorandum of understanding with the Tongan government to supply Tongatapu, the main island, and is targeting first power in 2027 with Leopard, a 2-megawatt commercial generator. Tonga is a sensible first market. Its electricity is among the most expensive in the world, at roughly $0.35 per kilowatt-hour in 2021, and most of it comes from imported diesel. Fuel price shocks pass straight through to consumers, and the country has spent years trying to displace them with solar, batteries and efficiency programs. Two megawatts is small in absolute terms. At a 90% capacity factor it would produce about 15.8 gigawatt-hours a year, roughly a quarter of Tonga's electricity sales, which run around 60 GWh annually. For a single generator, that is material. The economics will be decided by the cable. Endurance has not published the route, the length or the cost of the subsea link to Tongatapu. The company says it will choose sites by optimising cable cost against resource quality and onshore demand. Some prospects are a few dozen miles offshore; others are hundreds. That optimisation is the business model. A subsea field with high-grade heat close to a diesel-dependent grid can compete. A field far from shore, or one that needs a long high-voltage cable and a new converter station, may not. > It is adding so much technological challenge to an energy production technology.— Emily Pope, senior fellow, Center for Climate and Energy Solutions Pope also argues that remote islands could meet their needs more easily with solar and batteries. That is the competitive threat in its simplest form: a mature alternative that a utility can finance today, against a novel baseload source that has run for seconds. The addressable market is large if the model works. Andrew Redd, the founder and chief executive, estimates about 6 terawatts could be developed around the Ring of Fire within five to ten years. That is a company estimate, not an independent one, and it assumes costs fall along a curve that has not yet started. ## Terrestrial geothermal just set a harder benchmark Endurance is not competing with diesel alone. It is competing with geothermal on land, which has just had its own commercial breakthrough. Fervo Energy declared commercial operation at Cape Station in Utah on 30 September, the first greenfield enhanced geothermal project to reach contractual commercial operations. Fervo drills deep horizontal wells and fractures hot rock, borrowing the oil industry's playbook. Its costs are falling as it repeats the process. The cost trend is not uniformly friendly. IRENA's 2025 cost report put geothermal LCOE up 53% year on year, even as utility-scale battery costs fell about 30%. Enhanced geothermal well costs at some US projects have fallen by more than half, and the IEA projects next-generation costs could fall up to 80% as drilling matures. | Dimension | Terrestrial EGS | Subsea geothermal | | -------------------- | ---------------------------------- | ---------------------------------- | | Heat access | Deep vertical and horizontal wells | Shallow high-grade vents | | Permitting | Land, water and seismic consent | Marine and seabed consent | | Transmission | Grid connection on site | Subsea cable to shore | | Repair | Truck and workover rig | Ship and remotely operated vehicle | | Demonstrated runtime | Commercial operation | Seconds | Qualitative comparison. Fervo's Cape Station reached commercial operation on 30 September 2026; Endurance's Adélie ran net-positive for 8.7 seconds on 27 September. The table is unkind to the subsea case on transmission and repair, and generous on heat grade and siting. Endurance's answer is that the ocean offers high-temperature reservoirs close to coastal demand, without the land assembly and permitting that delay onshore projects. ## What would make this investable Three numbers will decide whether Endurance becomes a power producer or a science project. The first is run length. Adélie ran for seconds. Gentoo, the upgraded unit planned for this winter, needs to run for days, then weeks, without a support vessel alongside. The second is installed cost per kilowatt for a full 2-megawatt unit, including the well, the plant and the cable. Published offshore geothermal projections have used figures around $4,000 per kilowatt, an overnight cost that would make the technology competitive with new firm capacity if it holds. No commercial subsea unit has been built, so the number remains a projection. The third is availability. A subsea plant that needs a ship every few months will not beat diesel. Endurance's 16-hour turnaround is a good sign; a full maintenance campaign that takes weeks is not. #### Diligence checklist for the 2027 Tonga milestone Four disclosures would separate a commercial pilot from a demonstration: the Tonga cable route and its cost per kilometre; the measured capacity factor of Leopard over a full quarter; the number of wells drilled per successful producer; and the contracted power price against the diesel benchmark it replaces. **What would kill the case:** a cable cost that exceeds the plant cost, a well success rate below one in two, or a maintenance interval measured in months rather than years. ## The bet, stated plainly Subsea geothermal is a bet that the ocean's heat is easier to reach than the land's politics. The physics is real. The engineering is now demonstrated at the smallest possible scale. What remains is the unglamorous part: deeper wells, longer runs, cheaper cables and a repair record. Endurance has two years and one commercial project to show that those can be bought, not invented. 🎯 Adélie proves subsea geothermal can produce net power. It does not prove it can produce power on a schedule, at a cost below diesel. The binding constraint is well flow, not vent temperature. A 16-metre well could not run the plant; the milestone came from a natural vent. Tonga is the real test. A 2 MW unit at a 90% capacity factor would cover roughly a quarter of the country's electricity sales, if the cable economics hold. ### What happens if Gentoo clears a week of continuous operation? 🔮 **Endurance deploys a 2 MW unit in Tongan waters by the end of 2027, and the first year of operation reveals a capacity factor above 80%.** Probability: 45% — the engineering path is credible, but the cable, permitting and well-flow risks are all still open. #### 🟢 Optimistic scenario (30%) Gentoo runs for weeks, deeper wells deliver commercial flow, and the Tonga cable is financed as infrastructure. Endurance books a second island market and raises a project-finance facility in 2028. **Consequence:** subsea geothermal becomes a niche but bankable baseload product for island grids. #### 🟡 Base case (50%) The 2027 Tonga unit arrives late and runs intermittently. Endurance proves the concept, but the installed cost stays above diesel and the company needs another equity round before a second site. **Consequence:** a credible technology roadmap with a longer, more expensive path to scale. #### 🔴 Pessimistic scenario (20%) Well flow stays marginal, a subsea repair campaign stretches into months, and the cable cost breaks the Tonga business case. The company pivots to selling components or data. **Consequence:** the technology remains a research platform rather than a power business. ## Sources [ Endurance Energy: first net-positive subsea geothermal run The primary record of the Adélie test: well depth and temperature, vent temperature, net output, the failures that ended the dive, and the Tonga timeline. Endurance Energy ](https://www.enduranceenergy.com/press/adelie?ref=nexi.fund) Every operating number in this piece traces back to this release. Read it before believing any summary, including this one. [ IEEE Spectrum: testing geothermal power on the seafloor Independent engineering context on the organic Rankine cycle, the cabled observatory, subsea cable costs and the scepticism the approach attracts. IEEE Spectrum ](https://spectrum.ieee.org/offshore-geothermal-energy?ref=nexi.fund) The clearest explanation of why the cable, not the turbine, is the commercial question. [ GeekWire: a brief milestone for undersea geothermal Business reporting on the 100 kW test unit, staffing, corrosion and repair risk, and the Gentoo deployment planned for this winter. GeekWire ](https://www.geekwire.com/2026/endurance-energy-hits-a-brief-milestone-in-its-quest-for-undersea-geothermal-power/?ref=nexi.fund) Useful for the operational detail the press release omits, including the roughly three-dozen-person team. [ Climate Solutions: doubts over the economics The counter-case, including the 60 watt-hour total, the case for solar and batteries on islands, and expert scepticism about saltwater operations. Climate Solutions News ](https://climatesolutions.news/spotlight/endurance-energy-subsea-geothermal-net-power?ref=nexi.fund) The most direct statement of the bear case. Any valuation of Endurance has to answer it. [ TechCrunch: Endurance raises $54M for ocean geothermal Funding composition, the Founders Fund-led investor group, the founder's SpaceX background and the Ring of Fire resource estimate. TechCrunch ](https://techcrunch.com/2026/06/11/endurance-energy-raises-54m-to-harness-a-massive-untapped-energy-source?ref=nexi.fund) Background on the capital stack and the 6-terawatt claim that underpins the long-term case.