Reports from several sources are providing information about craters forming in the frozen north of Siberia over the last three years, some of them associated with the sound of explosions, which light up the sky. The most likely cause of this is global warming, which is melting the ice and releasing the melting methyl hydrate, which has previously been trapped underground.
Satellite images suggest a link with pingos which form when ice is trapped between layers of earth. See: http://uk.businessinsider.com/russian-exploding-permafrost-methane-craters-global-warming-2016-6?r=US&IR=T
Whatever the cause, it is a reason for great concern, for the release of methane into the atmosphere is going to significantly speed up global warming.
The following photograph was released by the Siberian Times:
Once the craters have formed it would appear that they grow in size and fill up with water.
There have been several reports on the phenomenon, with photographs, as follows:
Daily Mail ; Business Insider; National Geographic; Live Science; Siberian Times; Science Alert; slate.com; CNBC; Huffington Post etc