Siberia has recently played host to dozens of mysterious craters, the first of which appeared last July.

At the time, it was suspected that they were formed by the volatile release of methane hydrate as it melted from the Arctic ground.

Methane hydrate is comprised of a methane molecule surrounded by water molecules. The entire structure forms a crystal structure and is frequently found in the permafrost layer.

A new satellite survey has thrown this theory into dispute, however. The craters are now increasingly linked to melting of ice cores, rather than of methane hydrate.

The cores, called pingos, form near the surface of Arctic ground. When they melt quickly—as has been the case in Siberia in recent years—the surrounding ground can cave in, creating a crater.

“The jury is still out” on the definite cause of the craters, said Carolyn Ruppel, chief of the U.S. Geological Survey’s Gas Hydrates Project.

The process of rapid ice core melting does not account for rocks lining the edges of the craters, which suggest an explosive action.

Ruppel and team hypothesize that the weight of pingos has—until recently—kept stores of natural gas under pressure in the permafrost below.

When the ice cores melted, she suggests, the natural gas was explosively released to create the craters.

The satellite survey supports the suggestion: images from the past depict pingos in the precise locations of the current craters.

Moreover, Ruppel explained that methane hydrates, the prime suspect of July, are only ever seen in permafrost at least 225 meters (740 feet) below the surface. This is significantly lower than the craters, which average about 15 meters (50 feet) in depth.

Pingos are scattered across much of Siberia and have also been noted on Alaska’s North Slope. Ruppel cites these as likely to create additional craters across the Arctic landscape as the global temperature rises.

Until last week, only three craters had been discovered. Now satellites have revealed at least seven large craters (including one encircled by up to 20 lesser holes), according to National Geographic’s Brian Clark Howard.

Two of the previously discovered craters have since become lakes.

“We must research this phenomenon urgently, to prevent possible disasters,” said Vasily Bogoyavlensky of the Moscow-based Oil and Gas Research Institute.

According to Vladimir Romanovsky, a geophysicist at the University of Alaska Fairbanks, the craters could form anywhere with permafrost and sufficient stores of natural gas—such as parts of Alaska and northwest regions in Canada.

“If the warming continues,” Romanovsky said, “we will see more and more of this phenomenon.”