In following the excellent reporting of the recently observed craters that have formed in Siberia, I am convinced that these were formed by explosive release of Methane gas. If you have been following these reports, in my opinion, the best reporting has been in the Siberian Times.
The first report is here and includes excellent pictures. I agree completely with the expert Andrey Plekhanov quoted in this first report. Because of production activities in the Yamalo-Nenets Autonomous Region, some 30 kilometers from the Bovanenkovo gas field, changes in this system as a result of gas production might be a possibility, although this is difficult to determine without more detailed information. A youtube video of the site, collected by Russian scientists, can be seen here -> Siberian crater.
There is a sense in this article that methane clathrate might be related to this crater formation. You can review methane clathrate definitions here. A summary specific the oceans can be found here. These should not be confused with cold seeps. A scientific paper that describes explosive release of such clathrates, following the last ice age, is located here–this might be behind a fee-wall; just google until you find something similar, these terms will get you started. An open source link to the abstract, including the authors contact information can be found at this Harvard University site. Bottom line–the structures with respect to crater topology are identical with the Siberian craters. Unsurprisingly these crater structures were hinted at in bathymetric charts from 1954 and more a more detailed translated report completed during the Soviet Union era.
The world’s largest crater like structures have been documented offshore of New Zealand and are described here.
All these observations lead to the question–how much Methane is being released by oceanic and land systems from geological partition of the earth system? Here is a cool site showing work being completed in the East Siberian Arctic Ocean,
Methane hydrates are commonly found in sub-oceanic settings. A possible link with the Deepwater Horizon tragedy was proposed and is documented here.