There's a scary answer for why Siberia is turning into Swiss cheese: Mother Nature has gas, so to speak, and we gave it to her.
That's the preliminary conclusion of scientists who have explored the mysterious holes that began popping up in Siberia beginning last month, some of whom have postulated that climate change may be a cause.
And given the right conditions, some scientists are concerned that something similar could happen in other places around the globe, although not likely in Times Square, Hollywood and Vine or your backyard.
The first Siberian crater appeared in mid-July, fascinating and terrifying locals who found the massive hole in the Earth. Mere days later, people discovered a second one. And then a third.
Russian researchers who have explored the crater sites now believe the long-frozen Siberian permafrost thawed due to increased temperatures, collapsed and let free methane gas trapped beneath, the team told the science journal Nature. The team tested the air near the bottom of the holes and discovered an unusually high concentration of methane.
"Global warming is happening, and it's exacerbated in the Arctic," Carolyn Ruppel, chief of the U.S. Geological Survey's Gas Hydrates Project, told NBC News. "And if this [the Siberian crater phenomenon] is what we think, that it's related to permafrost thaw, It's a very visible effect of what's happening to the Earth."
Ruppel has spent the past five years working on methane in the Arctic, though she has not visited the Siberian crater sites. She has never seen anything like the craters, she said -- a sentiment echoed by other top experts in the field.
Mother Nature has gas, so to speak, and we gave it to her.
The holes are likely the direct result of unusually warm 2012 and 2013 summers in the area of the craters, said the Russian crater research team that spoke to Nature.The team, led by Alexei Plekhanov of the Scientific Centre of Arctic Studies, said the past two summers were warmer than usual by about five degrees Celsius, thawing the long frozen earth -- but one or two hot summers aren't necessarily the result of global warming.
Other researchers went a step further, Nature reported, attributing the holes to a long-term thaw that's a result of global warming. The craters are physical manifestation of the damage we are doing to the Earth, they say.
"It’s a clear indication that something is happening to the Earth," Ed Dlugokencky, a top federal federal scientist who researches methane in the atmosphere at the National Oceanic and Atmospheric Administration, told NBC News.
Two-thirds of climate change is caused by human activity, Dlugokencky said, and "we don't see that changing. So we’ll see if these sorts of changes become a widespread phenomenon. It's certainly possible. I'm concerned about it."
In the Arctic, at least, where global warming is felt acutely, Ruppel of the U.S. Geological Survey is especially concerned: "Instead of a solid block of permafrost, you now have a hole where warm summer air can get in."
"It’s a clear indication that something is happening to the Earth."
If holes develop below buildings and bridges in the Arctic, or near the abundant natural gas lines in the area, Dlugokencky noted, "that could have a devastating effect."
Of course, not all areas mirror the Arctic's permafrost- and natural-gas-heavy geology -- so the physical effect on other parts of the Earth may not be holes but something else entirely.
"What we may find a few years down the line is that these holes are a harbinger of things to come," Ruppel said.
In the meantime, Dlugokencky said, increased methane emissions at the crater sites are concerning enough. The scariest part about methane -- which has 20 times the effect on global warming as carbon dioxide does over a 100-year period -- is its cyclical damage, Dlugokencky said.
When methane is released into the atmosphere, it warms up the planet. And that warming, in turn, thaws permafrost and releases more methane -- starting the cycle anew.