60 Watt-Hours on the Seafloor: Endurance's Offshore Geothermal Bid

A 3-ton subsea power plant produced roughly 60 watt-hours of electricity in about 29 seconds last month, and the startup behind it is calling that a milestone. Seattle-based Endurance Energy announced the result Monday from a September demonstration at Axial Seamount, an active submarine volcano about 300 miles west of Oregon.

The numbers are small. The implications, the company argues, are not.

Adélie, the test unit, sat roughly a mile below the Pacific surface during a three-week expedition aboard the Ocean Guardian vessel. On Sept. 27, its turbine generated power from a natural hydrothermal vent. For nearly nine of those seconds, output exceeded the plant's own operating load, with the surplus charging a battery backup. The run ended when the battery's overcurrent protection tripped, and a power cable failed soon after, closing the dive.

What did the demonstration actually prove?

Endurance drilled a relatively shallow well into a vent field at the base of the volcano using a modified geotechnical drill of the type offshore oil and gas developers use for core samples. It then lowered Adélie onto the seafloor, where a submarine robot connected the unit to the well. The plant itself contains an organic Rankine cycle system — standard equipment on many land-based geothermal projects — that transfers heat from hydrothermal fluid to spin a turbine.

Notably, the well did not deliver. Hot, mineral-rich seawater flowed from it, but Adélie could not produce power from that source, and the team moved the unit to a nearby natural vent instead. A crew in Seattle monitored real-time temperature and flow-rate data and operated the plant remotely over an existing fiber-optic cable network.

Andrew Redd, founder and CEO of Endurance Energy and a former SpaceX engineer, framed the result as proof of concept rather than performance. "We follow the adage 'make it exist, then make it good,'" he said. "Adélie made it exist, and now we know it's possible."

He added: "This was our first end-to-end demonstration of subsea geothermal as a concept, and we learned just an incredible amount from every stop along the way."

How does this fit the broader geothermal push?

The Adélie test is an outlier in the effort to expand U.S. geothermal capacity using next-generation technologies in unconventional locations. Most developers are working terrestrially. Redd, who launched the company in January 2025, sees the most promise in volcanically active regions where tectonic plates spread apart.

Existing offshore geothermal initiatives have stayed in the research lane. In 2016, a team of Chinese scientists produced a few watts of sustained power from a hydrothermal vent where the southwest Indian Ocean meets the southeast Atlantic. In southwest Iceland, a government-backed project drilled a geothermal well near the Mid-Atlantic Ridge, but corrosive seawater created major engineering hurdles.

Endurance says it differs on scale and on design. The company is building its technology, in Redd's words, "from scratch" to survive deep-sea pressure and corrosion, using a lot of titanium.

Can the economics work?

Not everyone is persuaded. Emily Pope, a geologist and senior fellow at the Center for Climate and Energy Solutions, called it "an eyebrow-raising approach." Offshore geothermal is scientifically intriguing, she said, but hard to reconcile with the cost and complexity of operating in harsh, salty conditions and transmitting power over long distances to customers.

Even the apparent target market looks shaky. Remote island nations, seemingly ideal candidates, could adopt lower-cost solar and batteries far more easily. "It is adding so much technological challenge to an energy production technology, without there being a really significant demand for it," Pope said of the subsea pursuit.

The company's own pipeline points to that market anyway. Endurance has raised $63 million from venture capital firms and is partnering with the Kingdom of Tonga on a 2-megawatt project intended to reduce the island nation's reliance on imported diesel.

What happens next?

Endurance is "rearchitecting" core components of Adélie and plans to redeploy the refurbished unit this winter, aiming for higher power output over a longer run. In parallel, the company is developing a heavier commercial-scale system that Redd suggested could start generating electricity in 2027.

The site itself carries advantages. Axial Seamount sits on the Juan de Fuca Ridge and is a focus of oceanographic research for the University of Washington and the National Science Foundation's Ocean Observatories Initiative, which Redd said made it an ideal place to operate and monitor environmental conditions.

First, though, the endurance test: the technology must run for more than half a minute.