But Romanovsky said the hole doesn't look like a typical collapsed pingo; such features usually form from larger mounds that slowly cave in over a period of decades, with all the material falling inside.
From the photo of the Yamal crater, "it's obvious that some material was ejected from the hole," Romanovsky said. His Russian colleagues who visited the site told him the dirt was piled more than 3 feet (1 m) high around the hole's edges.
The crater's formation probably began in a similar way to that of a sinkhole, where water (in this case, melted ice or permafrost) collects in an underground cavity, Romanovsky said. But instead of the roof of the cavity collapsing, something different occurred. Pressure built up, possibly from natural gas (methane), eventually spewing out a slurry of dirt as the ground sunk away. Anna Kurchatova, a scientist at the Sub-Arctic Scientific Research Center in Russia, made a similar observation to The Siberian Times.
The photo of the crater rim shows some vegetation that does not appear freshly grown, which suggests the hole may be several years old, Yoshikawa said. Romanovsky said it might be more recent, but investigators will need to look at archived high-resolution satellite images to pin down exactly when the crater appeared.
And many other questions remain: If a sinkhole erupted material, why is the hole's border so round and even? Would there be enough gas to fuel such an eruption, and where did such gas come from?
This part of Siberia contains deep gas fields, and it also contains a lot of small lakes, which formed between 4,000 and 10,000 years ago when the climate was warmer, Romanovsky said. Perhaps these odd holes developed in the same way that sinkholes did, but later expanded.
Domes of natural gas also exist in the United States, located east ofthe Sagavanirktok River in Alaska's North Slope Borough.
The development of permafrost sinkholes could be one indication of global warming, Romanovsky. "If so, we will probably see this happen more often now."
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