The first time Endurance Energy's equipment touched down on a submarine volcano, the water was hot enough to melt lead. Four thousand feet below the Pacific's surface, somewhere off the coast of Tonga, superheated fluid gushed from a seafloor vent at temperatures north of 570 degrees Fahrenheit. The small Seattle startup had barely been in business a year.
Now, Endurance has something more terrestrial to show for its deep-ocean gambit: $25 million to $30 million in fresh funding. Founders Fund led the seed round, with sources familiar with the deal placing the final figure somewhere between $25 million and $30 million. It's an unusually large seed—perhaps more than the founders expected—for a technology most investors might classify as wildly speculative.
But that's the point, really. Endurance isn't drilling on land or chasing incremental efficiency gains. It's building modular generators designed to sit directly atop hydrothermal vents along the Pacific Ring of Fire, extracting energy from geological fury that's been boiling away, untapped, for millennia.
Mining heat from the abyss
Andrew Redd, Endurance's CEO and a Princeton graduate who spent four years pulling oars as a heavyweight rower, describes the vision in almost utilitarian terms: find a vent, clamp on a wellhead, pump the fluid through thermoelectric generators, and send baseload power up to the surface. His company claims the approach could eventually support "gigawatt installations" running around the clock—no sun required, no wind needed, no batteries involved.
The technical details remain thin. What Endurance has disclosed publicly, through a handful of company posts between April and May, is that it's deployed equipment on four separate submarine volcanoes since its founding around fifteen months ago. One prototype, according to a third-party observation on LinkedIn from late spring, operated at roughly 1,500 meters depth. The "wellhead prototype" the team describes can apparently mate to a vent, actively pump hydrothermal fluid, and power its own sensors and communications using onboard solid-state thermoelectric units.
Whether that prototype can scale to commercial relevance is another question entirely. Endurance hasn't released power output numbers, mooring schematics, or much detail on how it plans to transmit electricity from the seafloor to shore. The startup is, for now, heavy on ambition and light on data sheets.
Tonga takes a chance
Still, at least one government is willing to bet on the concept. In late February, Tonga's Prime Minister's Office signed a memorandum of understanding with Endurance to explore subsea geothermal energy. The partnership aligns with Tonga's policy goal of achieving 100 percent renewable electricity by 2035, a target laid out in its TERM Plus roadmap. Testing began in April in the Lau Basin, a seismically active stretch of ocean floor dotted with hydrothermal vents.
Local press coverage suggested the project could eventually replace diesel-fueled baseload generation—a significant cost burden for an island nation dependent on imported fuel. Tonga's Prime Minister was quoted expressing cautious optimism about shifting away from diesel, though the pilot remains just that: a pilot. The company has described the work in Tongan waters as its first deep-sea deployment in the region, a milestone that came roughly a month before early June.
Redd's team has grown to somewhere between 11 and 50 employees, according to LinkedIn, and the startup is actively hiring. Job postings in May and June called for a Principal Geologist, Environmental Specialist, Principal Turbomachinery Engineer, and Supply Chain Lead—roles that suggest the company is serious about moving beyond prototypes.
A hot moment for geothermal
Endurance is entering a market suddenly flush with capital and enthusiasm. Next-generation geothermal technologies, once dismissed as niche or quixotic, are drawing serious attention from hyperscale data center operators desperate for reliable, carbon-free electricity that doesn't depend on the weather.
In March, Ormat Technologies inked a geothermal power purchase agreement supporting Google's Nevada operations. Around the same time, the Department of Energy opened applications for $171.5 million in field-scale geothermal funding, with a deadline in April. Fervo Energy, which uses enhanced geothermal techniques on land, closed a $462 million Series E in November. Sage Geosystems raised $97 million in January with reported interest from Meta-linked offtake agreements. GA Drilling announced a $44.1 million round in mid-March.
Those companies, however, focus on onshore drilling or advanced subsurface engineering. Endurance appears to occupy its own category—literally and figuratively offshore. As of early June, no directly comparable, well-funded commercial player was identified in available reporting. That could mean Endurance is pioneering a new frontier. Or it could mean the technical and economic hurdles are steep enough that few others are willing to follow.
What comes next
Endurance hasn't said much about how it plans to spend the seed capital, and Founders Fund remains the only confirmed investor. Job postings have mentioned First Round Capital and Point72 Ventures, but those references remain unverified as of early June.
The startup is targeting island nations, industrial facilities, and hyperscale data centers—markets where baseload power is scarce or expensive. With testing underway off Tonga and a team scaling across engineering and environmental disciplines, Redd and his colleagues are betting that subsea geothermal can carve out a role in the broader renewable energy landscape.
But the hard questions remain unanswered. How efficiently can the technology convert geothermal heat into electricity at depth? What does the mooring and transmission architecture look like at scale? How will environmental impacts be monitored in some of the most remote and ecologically sensitive waters on Earth? And perhaps most crucially: can the economics work?
For now, Endurance is asking investors—and at least one Pacific island nation—to take it on faith. In the volatile, high-stakes world of climate tech, that might be enough to get started. Whether it's enough to reach commercial scale is a question that will be settled several thousand feet below the surface.
