Strange-shaped sea mountain discovered off the coast of Northern California

A strange-shaped, 3,300-foot-tall underwater volcano has been discovered just 184 miles off the Northern California coast.

The sea mountain, or "seamount," appears more like a smooth-sided circular tower, with near-vertical sides, than a craggy mountain. It was found in February by an ocean mapping autonomous sailboat as part of a multi-agency survey led by the National Oceanic and Atmospheric Administration.

"Typically seamounts have sloped sides, like Mount Fuji," Dr. Aurora Elmore, the program manager for NOAA's Ocean Exploration Cooperative Institute, told SFGATE. "But what's interesting about this one is that it's really steep. It rises from the bottom of the seafloor with a tower shape."

Elmore thinks this unique shape may be because the volcanic activity that formed it was "super hot, and happened all at once." Alternatively, the structure may have lost a more gradually sloped base due to a build up of sediment known as "marine snow," or as Elmore puts it: "Millennia of fish poop."

The find is an anomaly for another reason, too. While there are 63 identified seamounts off the California coast, there are none near the one found off the coast of Cape Mendocino. "It doesn't fall into the area of known seamounts," Elmore says. "It expands the range."

The 1,200-foot-deep crater at the summit of the new underwater volcano sits about 2 miles below the ocean's surface, and was found through the use of a vessel operated by Bay Area ocean research company Saildrone, in partnership with NOAA and other agencies.

The orange-sailed uncrewed sailboat Surveyor SD 1200 surfs the ocean using a multi-beam sonar on its hull to map the contours of the depths below. The vessel is controlled remotely by pilots at the company's Alameda base.

"The Surveyor weathered the storms, collected high-resolution bathymetry, and put no humans at risk," Saildrone vice president Brian Connon said in a statement. "... This is the future of ocean mapping."

The vessel set sail from Alameda in July 2022 and mapped Alaska's Aleutian Islands before heading back down the California coast.

The unique underwater discovery is making waves in the oceanography community. Elmore says that only 50% of the U.S. exclusive economic zone — an area of the oceans within 200 miles of any U.S. coastline — has so far been mapped this way.

"The immediate reaction from the team was that it looked like a Bundt cake," Neah Baechler, lead surveyor for Saildrone, told the Sacramento Bee. "It's very round and with steep sides and a curved top that slopes into a crater in the center."

Similar features have been found on the ocean floor, but rarely this big, or this complete. "The Bundt cake shape is not unique to this feature, although finding one large enough to qualify as an official seamount is less common," Baechler told SFGATE over email. He described a much smaller previous find, in a C-shape — "More of a Bundt cake with a few slices missing," Baechler said.

Seamounts are the remnants of extinct volcanoes formed millions of years ago on the ocean floor, NOAA says, with a height from seafloor to summit of at least 3,300 feet. The underwater mountains will occasionally breach the water's surface, giving rise to islands formations, like Hawaii.

California's new underwater mountain does not yet have a name; that process will soon be completed by the General Bathymetric Chart of the Oceans' Sub-Committee on Undersea Feature Names.

Elmore hopes one day to send a remotely-operated video camera into the ocean, to get a closer look at the underwater tower, standing taller than Mount Tamalpais off the California coast. "They are a weird and wonderful feature on the bottom of the seafloor," Elmore says.