So many craters!
So many craters! K. Andreasson/CAGE

In the polar reaches of the Arctic Ocean, in northern Bjørnøyrenna, the seafloor is bumpy, pockmarked mess. Much of the seabed in the region is smooth, but not in the area that’s the subject of a new paper published in Science, where it is dotted with giant mounds and craters.

Karin Andreassen, of University of Tromsø - The Arctic University of Norway, didn’t expect to observe many features there, according to The Atlantic. But when she and her team studied the area, they found more than a hundred giant craters, ranging from about a fifth-mile to a half-mile wide. They were deep, too, about 100 feet into the seafloor. They “generally have an oval shape,” Andreassen and her colleagues write in their paper. On the edges of the craters, the team found strange mounds of similar size and shape—more than a half-mile on a side, about 65 feet high, and semicircular or elliptical in shape.

What had happened to give the seafloor such weird goosebumps?

The remains of methane burps.
The remains of methane burps. K. Andreasson/CAGE

In the paper, Andreassen and her colleagues describe a likely origin: methane burps. Thousands of years ago, this area of the ocean was solid ice, and as movements of the Earth below pushed methane up, the gas would have been frozen in the bedrock below the ice.

Then conditions changed. The ice sheet melted away, and that “the rapid transition of the seafloor from a subglacial to marine environment,” the scientists write, “degraded conditions conducive to hydrate stability.” In other words, the methane turned back to gas and accumulated closer to the surface of the seafloor. In some cases, the gas caused the bedrock to buckle up into the giant mounds. In other cases, the pressure burst the mounds open and created craters.

Similar structures have been found on land, in the melting permafrost in northern regions. Methane is a potent greenhouse gas, and if it’s released into the atmosphere by a warming climate (from land or the bottom of the ocean), it could speed up an already runaway process.