Scientific Centre of Arctic Studies in Salekhard. The newly discovered Siberian crater is filled with methane, indicating it was caused by the release of gas from thawing permafrost.

Air samples taken at the bottom of one of the craters that have recently appeared in Siberia seem to support fears that the hole was formed by methane released from melting permafrost. If so this is very bad news for the planet's future, indicating frighteningly high emissions of the powerful greenhouse gas.

An expedition from the Scientific Center of Arctic Studies found methane concentrations of 9.6% at the bottom of the crater – 50,000 times the atmospheric average. The possibility that methane released by melting permafrost produced the crater had been a favored hypothesis from its discovery in mid-July. Nevertheless, plenty of other theories were circulating, and scientists urged caution before leaping to conclusions.

The extraordinary concentration of methane, on the other hand, seems unlikely to be a coincidence, particularly since methane is slightly lighter than air. The 2012 and 2013 summers on the Yamal peninsular, where the crater is, were around 5°C warmer than normal.

Expedition leader Andrei Plekhanov told Nature that the high temperatures probably thawed the permafrost to the point where it collapsed, releasing the trapped methane.

A competing theory holds that a single warm season, or even two, would not be enough to create such a deep hole. Hans-Wolfgang Hubberten of the Alfred Wegener Institute points to a 2°C warming at a depth of 20 meters in Arctic permafrost as a result of Global Warming over the last 20 years.

Hubberten's theory is that a thick layer of ice trapped methane released as the soil below thawed. “Gas pressure increased until it was high enough to push away the overlying layers in a powerful injection, forming the crater,” he told Nature.

While Hubberten says he has never seen a crater like this before, two smaller ones have been reported in the weeks since it was found. This does not mean that these craters have formed in that time. One was seen to form in September 2013, although eyewitness accounts of what happened vary. It is not known when the other appeared.

Establishing the method by which the crater forms is important, as it might provide ways to warn local villagers, or the operators of the nearby Bovanenkovskoye gas field, if a crater is about to appear somewhere dangerous.

The most serious question is what this means for the release of methane from the tundra. Methane is a much more potent greenhouse gas than carbon dioxide – traditionally estimated as 25 times, but the most recent IPCC report raised this to 34 times over a 100 year period. The possibility of warming releasing Arctic methane has been the most publicized of the possible scenarios for runaway climate change even if human emissions become controlled, although some have questioned the doomsday scenarios.

Plekhanov is keen to return to the crater to measure methane trapped in the walls, both to settle the question of the process that led to its formation and gain an idea of how much methane is being released.