Dr Semiletov's team published a study in 2010 estimating that the methane emissions from this region were about eight million tonnes a year, but the latest expedition suggests this is a significant underestimate of the phenomenon.

In late summer, the Russian research vessel Academician Lavrentiev conducted an extensive survey of about 10,000 square miles of sea off the East Siberian coast.

Scientists deployed four highly sensitive instruments, both 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 10,000 square miles, 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," Dr 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 a kilometre or more wide and the emissions went directly into the atmosphere – the concentration was a hundred times higher than normal."

Dr Semiletov released his findings for the first time last week at the American Geophysical Union meeting in San Francisco.