Many volcanoes are among the most majestic sights on the planet: Tall and wide, they belch molten rock or plumes of ash that can tower miles high. But there’s another class of volcano that’s much less impressive. These guys are short and squatty. And they burp out bubbles and blobs of mud, water, and gas. What they lack in majesty, though, they make up for in numbers: more than a thousand have been discovered on land, and many others have been found at the bottom of the ocean.

One of the most recently discovered marine examples is the Borealis Mud Volcano. It’s a quarter of a mile deep in the Barents Sea, an extension of the North Atlantic Ocean, between Finland and Greenland. It’s a little more than 20 feet wide and about eight feet high. It’s in a large field of craters and small cones. Like Borealis itself, many of the cones put out plumes of methane gas.

Geologists discovered Borealis during a 2023 research cruise. They studied it in more detail a year later. They suggest that it formed about 18,000 years ago, at the end of the last ice age.

During the ice age, what is now the ocean floor was covered by a giant slab of ice. As the ice melted and retreated, the pressure went down and the temperature went up. That allowed a big pocket of methane ice to begin vaporizing. The pressure blew a hole in the sediments above it. Today, the Borealis Mud Volcano continues to belch out methane, mud, and water from below the Barents Sea.

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