It was aliens, or maybe a meteorite or a gas explosion.
Those were some of the initial explanations for why three sinkhole-like craters gaped open in the Siberian permafrost, spawning a wave of Internet videos and frightening headlines about catastrophic releases of methane from a "dragon breath" of the Earth. Now, some scientists who have visited the holes, or have just monitored them via media reports, say there may -- with an emphasis on may -- be a warming component to their formation.
Others, however, say they may be the result of a natural process, at least in part. The bottom line is that no one knows the definitive cause of the massive craters turning Siberian permafrost into a Swiss Cheese slice.
"I have an opinion, but no proof," said Marina Leibman, a scientist at the Russian Academy of Sciences, in a brief email interview this week. In mid-July, Leibman led a team of scientists to the site of one of the holes that was approximately 30 meters in diameter and about 70 meters deep and was located about 30 kilometers from the Bovanenkovo gas field.
The holes appeared in the Yamal Peninsula, a reindeer-herding area holding the country's largest natural gas reserves.
In Leibman's view, an unusually warm summer in the Yamal region in 2012 caused extensive permafrost melt, which unleashed methane gas trapped in the ice and sediment. That expanding gas presence unleashed by melt in turn caused the permafrost to pop up like a cork, spraying debris in a visible ring around the created hole, she explained. The presence of the debris likely couldn't have happened without such a popping action, she said.
It was not an explosion, but more of an eruption that sprayed debris far from the hole itself. The visited chasm most likely formed in 2013, according to Leibman. The holes have to be relatively recent, because of vegetation patterns, further indicating that the warm summer of 2012 may have played a role, she said.
Despite concerns about methane release from permafrost generally, the 30-meter-wide (98.4 foot) funnel is not likely to be a constant source of gas after the initial blowout, according to researchers.
"Sure some methane was released as it is a reason for the whole thing ... but when we were there we measured methane content and it was higher than the normal but far from a limit of risk of explosion," Leibman said. When asked about a climate change link, she said she preferred the term "local climate fluctuations."
She said she hoped to return to the site, as "there is a good chance that the process will expand."
Much of what is known about the crater basics came from The Siberian Times, which reported earlier this month the discovery of the first hole with an icy lake at its bottom in northern Siberia, and later posted a flood of photos and video of a flyover of the phenomenon.
Later, the Times reported the discovery of two smaller holes hundreds of kilometers from the first one -- one was near the village of Antipayuta, and the other was found by reindeer herders in the Taymyr Peninsula. A scientific team from the Centre for the Study of the Arctic and one from the Russian Academy of Sciences were dispatched to the site, according to the Times, to collect sediment samples and take measurements. They told the paper there was no evidence of a meteorite strike.
'A mystery to me'
The initial photos spawned a wave of speculation on the Internet, irking some scientists along the way. Jason Box, a professor at the Geological Survey of Denmark and Greenland, posted an article on his blog highlighting broad concerns about greenhouse gas release from melting permafrost around the time of the hole hoopla and was later linked on websites as attributing broad methane concerns to the Siberian craters.
In an email, however, Box said, "To be clear, I draw no association with Siberian holes and climate change. Why not? Because I have no real info/understanding of the holes." He published a follow-up blog post with the title "Siberian tundra holes are a mystery to me."
Other permafrost experts, meanwhile, say that there may be a climate change connection, considering the rise of permafrost and air temperatures in the region over the past 40 years. One possible explanation is that warming helped create a cavity underneath the ground covered by a relativity stable top, said Vladimir Romanovsky, a professor of geophysics at the University of Alaska, Fairbanks, who has spoken to Russian scientists investigating the holes.
In theory, such a cavity could be created underground over a period of years as pure ice deep underground became more vulnerable to melt than a mix of ice and earth material at the surface, which acted like a stabilizing roof. Those dynamics, combined with an underground water source at the spot -- also boosted by warming -- perhaps helped create the underground cavity.
When pressure built up in it from gas, it could have burst the top, he said.
The pressure could have come from methane either from melting surrounding ice or perhaps from a deep underground pathway transferring it from the area's rich gas fields, he said.
An explosive puzzle
What will be key to know, Romanovsky said, is whether the scattered debris came from the shallow area of the hole, or from a deeper area. If it came from a deep area, the theory of an existing cavity doesn't hold, he said. "We'll have to start over" in that case, he said. Russian scientists are analyzing the samples, and are checking remote sensing data from satellites and airplanes to determine what the area looked like beforehand, he said.
Wayne Pollard, a permafrost expert at McGill University in Canada, said he was skeptical that warming alone caused the phenomenon, although it likely played a role in terms of surface melt unleashing methane. It's possible that natural geology, such as a fault in the area, contributed to making the location unstable, he said.
Something at the site "probably would have happened anyway. It may have not have popped out, but seeped out more slowly" without the added factor of warming, he said.
It's also possible the holes are some variation of what is known as a collapsed pingo, when an ice-filled dome in the permafrost created by the refreezing of part of a drained lake melts in the middle, said Pollard. Usually that doesn't form a sinkhole, but perhaps it's something with a "similar process" in this case, he said.
Like Leibman, Pollard and Romanovsky said these types of holes are probably not a concern in terms of long-term methane release.
Other permafrost areas farther south, with extensive frozen wetlands vulnerable to melt, are probably more of a concern with methane, said Pollard. Last year, scientists analyzing a "methane bomb" study made a similar point, noting that the Arctic is still a smaller source of the greenhouse gas than natural processes generating it from tropical swamps (ClimateWire, July 25).
Pollard said the holes are significant as an example of a broader trend of a changing Arctic. Romanovsky. added that it's going to be important to examine remote areas of permafrost more closely to see if other similar holes exist. He said it's possible that some lakes, for instance, may originally have started as sinkhole-like formations.
There also is an issue with infrastructure in the gas-rich region, he said, as developers have had the comfort that they would have time to plan for infrastructure damages from slow melt.
"So in this case of a kind of sudden episode, it could be very damaging. If you don't expect it, you are in big trouble," Romanovsky said.
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