Aliens, government conspiracies, meteorites, missile explosions, or even an elaborate prank – these were some of the wild ideas buzzing around the discovery of a giant crater in Siberia in July. Days later, a second gaping hole was found. And then another. Despite being separated by hundreds of kilometres, they were similar in appearance and shrouded in the same mystery – what caused these curious sinkholes?

Scientists would have to wait before they could venture in to find out.

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The first crater (pictured in preceeding slides) lies on the northern tip of the Yamal peninsula, the term “Yamal” meaning “end of the world” in the local language. It was an unstable place to explore, or even approach too closely, with its rims melting and falling inside, and the sound of water running deep within. The marshy state of the cavernous hole meant scientists had to wait until winter, when the crater had frozen solid, to determine what was inside.

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In November, that time finally came. For the first time scientists, led by Vladimir Pushkarev, director of the Russian Centre of Arctic Exploration, were able to explore the chasm’s depths. With stunning photos of the expedition released, the world can now see what lies inside.

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The mission was led by Vladimir Pushkarev, director of the Russian Centre of Arctic Exploration. He and his team used climbing equipment to scale down the sides of the crater. At the bottom, they measured the depths of the frozen lake – it was at least 10.5 metres, but may be more. Working in temperatures of about -11°C, they made measurements and collected data, performing radiolocation tests and taking probes of the ice, ground, gases and air.

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The crater was the first of the three discovered – and by far the largest at about 30 metres wide and 16.5 metres from the crater’s rim to the surface of the frozen lake.

The second crater was also discovered on the Yamal peninsula, a few hundred kilometres southeast in a region known as the Taz district. It is 15 metres across and was discovered when local reindeer herders nearly fell in.

At a mere four metres in diameter, the third crater is significantly smaller than the other two – but a great deal deeper, believed to be anywhere from 60 to 100 metres. It was also discovered by local herders in the northern region of the Taymyr peninsula, east of the Yamal peninsula.

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One of the first plausible explanations for the craters was offered up by an Australian scientist who suggested the culprit might be “pingos”, which are large blocks of ice that form just beneath the Earth’s surface that can leave behind gaping holes in the ground after they melt away.

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But more favoured ideas point to a much more explosive origin – methane. The northern Arctic regions of Siberia are known to be very rich in natural gas, and scientists had measured extraordinarily high levels of methane within the crater right after its discovery. So one group of scientists suggested that permafrost in the area had thawed due to the last two abnormally hot summers, which released methane gas deposits trapped below the permafrost, causing an eruption.

Others dismissed the idea that two summers could cause such a phenomena, and instead pointed to long-term permafrost thaw induced by climate change that eventually led to enough melt for it to collapse. These pockets of methane trapped below the permafrost are known as gas hydrates. The crater is located where two tectonic faults intersect – and the faults have been active in this area, which could be a contributing factor.

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Scientists must first process and analyse the data collected from the site before they support any of the ideas and suggestions. Pushkarev said the team from the Russian Centre of Arctic Exploration would also explore the surrounding areas and examine photos taken from space, as it is possible that there might be other craters yet to be discovered.