Now, scientists at the Centre for Arctic Gas Hydrates, Environment and Climate (CAGE) at UiT - The Arctic University of Norway, have collected new geophysical data on the craters in the area.
But instead of finding just a few craters, researchers found hundreds of them. They range in size from 300 to 1000 metres in diameter. And they have been blown out of solid bedrock.
The craters are what remain after a methane gas explosion. Or more correctly: gas blow-outs.
The blow-outs may have occurred over a couple of days.
But these blow-outs must have involved enormous forces at the bottom of the Barents Sea, since the holes have been blasted out of solid bedrock.
The researchers in Tromsø expect further blow-outs, as some gas chambers have not yet blown, and others are slowly building up pressure again.
Andreassen thinks there will be more large blow-outs in the ocean floor south of Svalbard, as the seabed becomes warmer and gas-hydrate reservoirs thaw.
Andreassen says there is probably even more methane gas locked in the seabed around Greenland and in the Arctic Ocean than is found in the Barents Sea.
That could lead to some extremely large craters.
Researchers have now examined satellite images of northern Siberian from a few years back and looked at the area where the explosions occurred. They found that the year before the huge crater appeared, there were large pingos in the same place.
Russian scientists have now mapped 7,000 gas-filled pingos that are poking through the thawing permafrost, visible in satellite images that illustrate how the pingos form and grow.