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Arctic researchers have long documented how mounds of ice, buried in dirt and soil, can erupt from the permafrost. These humps, called hydrolaccoliths or, more frequently, pingos, have cores of frozen water that migrate upward. Such pingos are typically up to a kilometer in diameter and several tens of yards high.

But the smaller, methane-filled bulges do not fit the classical definition of a pingo, according to a permafrost decay expert who spoke to The Washington Post.

“This is really a new thing to permafrost science. It has not been reported in the literature before,” said Vladimir E. Romanovsky, a geophysicist at the University of Alaska in Fairbanks. “It doesn’t mean the process is new. It means that it was never picked up and reported on.” Perhaps for want of a better name, Romanovsky called the gassy bulges “alternative pingos.”

It is just a matter of time when some of those craters appear in North America as well.

When researchers drill straight down into a traditional pingo, they hit the kernel of ice at the center. Coming in from the sides, drillers might discover water, close to the freezing point or possibly supercooled. But if someone were to take a drill to an alternative pingo? “That’s not good news,” as Romanovsky put it. The gas within the hill is not only under immense pressure but is quite flammable.

These humps are some 50 to 100 yards across, up to a tenth of the size of a classical pingo. Romanovsky said it was unknown exactly how the alternative pingos were formed. But he has an idea. In place of an ice kernel, alternative pingos may contain methane hydrate, the milky solid that forms when water mixes with natural gas and freezes.