Endurance Energy says its prototype marine geothermal power system generated more electricity than it consumed during a recent deployment 300 miles off the Oregon coast — though the milestone lasted just 8.7 seconds.

The Seattle-based startup landed its 3-ton power plant on the seafloor near a natural vent releasing water at about 538 degrees Fahrenheit. The team operated the system remotely through a fiber-optic network running along the seabed.

The expedition marked Endurance’s fifth deployment to the ocean floor where it tested Adélie — a 100-kilowatt geothermal generator named for a penguin.

The company is part of a broader race to harness geothermal energy, using heat from water or steam under the Earth’s crust, converted into electricity. But while companies such as Fervo and Sage Geosystems focus on terrestrial sites, Endurance is the first commercial venture pursuing undersea sources.

“The main question we had is can the system generate more power than it uses,” Andrew Redd, Endurance’s founder and CEO, said in a statement about the expedition, which began last month.

“Adélie did,” he added. “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.”

The test produced only a brief burst of surplus electricity, reaching a peak net output of 16.5 kilowatts. Challenges with the technology and a failure of a power cable cut the dive short.

The team also drilled what it called the world’s first geothermal well. At a site about 5,000 feet below the ocean’s surface, its machinery created two bore holes that tapped into hydrothermal fluid. It capped one hole and drilled a roughly 2-inch wide, 52-foot deep geothermal well at the other. While the well delivered hot fluid, it wasn’t suited for testing the energy system, sending the team to the natural vent.

Endurance said the expedition included voluntary environmental monitoring. The team mapped the seafloor to select the least disruptive test site, sampled water before and after operations, surveyed marine life and monitored sound.

While the startup has made rapid progress, its approach faces significant challenges: high pressure and corrosive conditions, mineral buildup inside equipment and the risk of costly repairs to seafloor machinery. And although many population centers sit near coastlines, the systems would still need expensive subsea cables to carry electricity over potentially long distances.

The company has raised $63 million from investors and has roughly three-dozen employees.

This winter, Endurance plans to deploy Gentoo, a refurbished and upgraded version of Adélie designed to generate more power for longer periods.

The company is also developing its first commercial project in Tonga, with the goal of delivering its first electricity in 2027.

The world’s first demonstration of a subsea geothermal power plant has taken place near an active underwater volcano almost a mile below the ocean surface. It generated less than 10 seconds of net-positive electricity but showed the technical feasibility of tapping another form of renewable energy underwater.

The three-week test deployment of the 3-ton prototype power plant took place at the Axial Seamount in the northeast Pacific Ocean in September. Endurance Energy, the Seattle-based startup behind the effort, aims to eventually commercialize subsea geothermal power for Pacific island nations and other customers looking for alternatives to importing expensive fossil fuels.

“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,” said Andrew Redd, founder and CEO of Endurance Energy, in an email to Ars.

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