One of the greatest fears is that with the disappearance of the Arctic sea-ice in summer, and rapidly rising temperatures across the entire region, which are already melting the Siberian permafrost, the trapped methane could be suddenly released into the atmosphere, leading to rapid and severe climate change.
Semiletov's team published a study last year estimating that the methane emissions from this region were about 8 million tonnes a year, but the latest expedition suggests this is a significant underestimate of the phenomenon.
In late northern summer, the Russian research vessel Academician Lavrentiev conducted an extensive survey of about 25,900sq km of sea off the East Siberian coast. Scientists deployed four highly sensitive instruments, seismic and acoustic, to monitor the "fountains" - or plumes - of methane bubbles rising to the sea surface from beneath the seabed.
"In a very small area, less than [25,900sq km], we have counted more than 100 fountains, or torch-like structures, bubbling through the water column and injected directly into the atmosphere from the seabed," Semiletov said.
"We carried out checks at about 115 stationary points and discovered methane fields of a fantastic scale - I think on a scale not seen before. Some plumes were 1km or more wide and the emissions went directly into the atmosphere - the concentration was 100 times higher than normal."
Semiletov released his findings for the first time last week at the American Geophysical Union meeting in San Francisco.