The mysterious crater spotted in the frozen Yamal peninsula in Siberia earlier this month was probably caused by methane released as permafrost thawed, Russian researchers say.
"Air near the bottom of the crater contained unusually high concentrations of methane - up to 9.6 percent - in tests conducted at the site," said Andrei Plekhanov, an archaeologist at the Scientific Centre of Arctic Studies in Salekhard, Russia.
The air normally contains just 0.000179 percent methane.
In Siberian permafrost, large deposits of methane gas are trapped in ice, forming what is called a gas hydrate.
"Methane remains stable and frozen at certain temperatures, but as the permafrost warms and its internal strength decreases, it may be less able to withhold the build-up of sub-surface gases," Plekhanov added.
Various theories have been doing the rounds about how the 30-metre-wide crater was formed - a gas or missile explosion, a meteorite impact or an alien involvement.
Plekhanov and his team believe that it is linked to the abnormally hot Yamal summers of 2012 and 2013, which were warmer than usual by an average of about 5 degrees Celsius.
As temperatures rose, permafrost thawed and collapsed, releasing methane that had been trapped in the icy ground.
To confirm what caused the crater, Plekhanov says that researchers will return to check the methane concentration in air trapped in its walls.
"Its rims are slowly melting and falling into the crater. You can hear the ground falling, you can hear the water running, it's rather spooky," he said in a Nature report.
