The article hints that it's formed from gas pressure: > The craters appear because ‘gas-saturated cavities are formed in the permafrost… > "In a literal sense

The article hints that it's formed from gas pressure: > The craters appear because ‘gas-saturated cavities are formed in the permafrost… > "In a literal sense, a void space filled with gas with high pressure. The covering layer distends, the thickness of which is 5-10 metres approximately." > Explosions have happened in swelling pingos, or mounds in the tundra which erupts when the gas builds up under a thick cap of ice.