In this frame grab made Wednesday, July 16, 2014, Andrei Plekhanov, a senior researcher at the Scientific Research Center of the Arctic, stands at a crater, discovered recently in the Yamal Peninsula, in Yamalo-Nenets Autonomous Okrug, Russia. (AP / Associated Press Television)

The mystery of the giant, gaping holes in Siberia that have spawned many theories about their origins may have been solved (and spoiler alert, they were not caused by aliens).

After visiting the site of the first hole which was discovered last month by reindeer herders in the Yamal Peninsula, a team of Russian scientists believe the craters were likely caused by a buildup of methane.

According to the researchers, Siberia’s extremely warm summers over the past two years caused permafrost to melt. A chain reaction was then started as underground methane was released. Scientists say gas pressure likely built up to a tipping point until there was a sudden release, creating the cavernous sinkhole.

The Russian scientists’ findings were reported in the journal Nature.

A sensor that was sent down one of the holes, measuring about 60-metres wide, found an unusually high concentration of methane — approximately 9.6 per cent, as compared to the usual 0.000179 per cent. Scientists said the level of methane in the hole, paired with the fact that mounds of dirt circle the mouth of the sinkhole support their theory.

Since the discovery of the first giant crater in northern Siberia, two others have been spotted. Snapshots of the cavities have gone viral, sparking a number of theories about what the caused them. Explanations for the sinkholes have included everything from meteorites, climate change to an alien invasion.

Scientists said they plan on returning to the holes to conduct further research, however, they note that by the time they return, some of the evidence they will be looking for may no longer be there. They said the walls of the sinkholes are collapsing, and water movement along the bottom of the holes can be heard.

The depth of the enormous cavities has not been established yet. A camera was lowered down into the dark pit of one of the sinkholes, but its line was not long enough to reach the lowest point. A pool of water – likely from the melting permafrost – was discovered at the bottom. Researchers estimate the depth to the water is approximately 70 metres.