Methane gas is the sole excavator.
As the temperature rises
There'll be more surprises
As threats to the climate gets greater.
Permafrost is found widely within on high latitudes in places like Siberia - the term indication that the ground is permanently frozen. In many areas the ground contains large quantities of methane, in the form a solid methane hydrate. If the climate warms the ground starts to thaw and the methane hydrate breaks up, releasing methane gas in the process. As methane is a much more powerful greenhouse gas than carbon dioxide the effect could be to accelerate global warming and the fact that temperatures are rising in the arctic is a cause for concern.
The above picture shows a crater, photographed shortly after it formed in 2014, situated on the Yamal Peninsula of Northern Siberia. Subsequently more craters have been discovered. What is happening is that as the ground warms (but is still frozen) the methane hydrate decomposes and releases methane gas which cannot immediately escape. The pressure increases until it reaches a point where it causes an explosive outburst - producing a large crater.
For more information see: The Siberian crate saga is more widespread - and more scarier - than anyone thought.