The three mystery craters that appeared earlier this month in northern Siberia have triggered a number of theories about how they formed, including their creation by aliens and meteorites

Now one scientist believes he has a more concrete explanation. The craters, claims geophysicist Vladimir Romanovsky, were formed by a type of sinkhole that erupted outwards rather than collapsing inwards. His theory is backed up by recent helicopter footage of the first hole in the Yamal region which reveals a mound of loose dirt that appears to have been thrown out.

A sinkhole is a hole in the ground created by erosion and the drainage of water in an underground cavity. The water inside the first crater likely came from melting permafrost or ice, said Mr Romanovsky, who studies permafrost at the University of Alaska Fairbanks speaking to Tanya Lewis at LiveScience.

He explained that whereas most sinkholes suck collapsed material inside, ‘this one actually erupted outside.’ ‘It’s not even in the scientific literature. It’s pretty new what we’re dealing with,’ he added. Mr Romanovsky believe the crater’s formation began in a similar way to that of a sinkhole, when ice collected in an underground cavity.

Rather than the roof of the cavity collapsing, he believes pressure built up from natural gas such as methane. This eventually erupted out a slurry of dirt, creating a crater in its place. However, the theory does not explain why the hole’s border so round or where the gas came from to fuel such an eruption. Last week two similar craters were discovered. A second is in the same permafrost region of northern Russia, and a third on the Taymyr Peninsula, to the east, in Kransoyark region. Both were spotted by reindeer herders who almost fell in.

The original hole received worldwide attention after being identified by helicopter pilots some 20 miles (32km) from a huge gas extraction plant at Bovanenkov. ‘Theories range from meteorites, stray missiles, a man-made prank, and aliens, to an explosive cocktail of methane or shale gas suddenly exploding,’ reported The Siberian Times.

‘The version about melting permafrost due to climate change, causing a release of methane gas, which then forces an eruption is the current favourite, though scientists are reluctant to offer a firm conclusion without more study.’ The new Yamal crater is in the area’s Taz district near the village of Antipayuta and has a diameter of about 49ft (15 metres).

A deputy of the regional parliament - or duma - Mikhail Lapsui said: ‘I flew by helicopter to inspect this funnel’ which he said was formed last year though only now have reports of it reached the outside world. ‘There is ground outside, as if it was thrown as a result of an underground explosion. ‘According to local residents, the hole formed on 27 September 2013.

‘Observers give several versions. According to the first, initially at the place was smoking, and then there was a bright flash. In the second version, a celestial body fell there.’ The chief scientist of the Earth Cryosphere Institute, Marina Leibman, told URA.RU website in Sibera: ‘I have heard about the second funnel on Yamal, in Taz district, and saw the pictures.

‘Undoubtedly, we need to study all such formations. It is necessary to be able to predict their occurrence. ‘Each new funnel provides additional information for scientists.’ The third crater and hole is in the Taymyr Peninsula and was accidentally discovered by reindeer herders who almost fell into it, in the vicinity of the remote outpost of Nosok.

The funnel is a perfectly formed cone, say locals who are mystified over its formation. Its depth is estimated at between 200 to 330ft (60 to 100 metres) and its diameter - more than 13ft (four metres). Experts - geologists, ecologists, and historians - have not come to a consensus about the origin of the funnel, say reports in the region. ‘It is not like this is the work of men, but also doesn’t look like natural formation,’ said one account expressing puzzlement at its creation.

Scientists in Krasnoyarsk region - the second largest in Russia - plan further study of this hole.  ‘It is not as wide as aerial estimates which earlier suggested between 164ft and 328ft (50 and 100 metres).’ Andrey Plekhanov, senior researcher at the Russian Scientific Centre of Arctic Research, revealed that satellite mapping imagery is being used to establish when the phenomenon was formed, thought to be in the last year or two.

‘The crater has more of an oval than a circular shape, it makes it harder to calculate the exact diameter,’ he said. ‘As of now our estimates is about 98ft (30 metres). If we try to measure diameter together with soil emission, the so-called parapet, then the diameter is up to 197ft (60 metres).

The structure is so fragile that the scientists could not climb deep into the lake and had to send a camera down instead.

One theory is that the feature is a ‘pingo,’ reports the Sunday Morning Herald. A pingo is a large chunk of ice that is located underground that can create a hole in the ground when it melts.

‘Certainly from the images I’ve seen it looks like a periglacial feature, perhaps a collapsed pingo,’ Dr Chris Fogwill of the University of New South Wales said. ‘This is obviously a very extreme version of that, and if there’s been any interaction with the gas in the area, that is a question that could only be answered by going there.’

Dr Plekhanov added the hole was most likely the result of a ‘build-up of excessive pressure’ underground, due to the region’s changing temperatures. He said 80 per cent of the crater appeared to be made up of ice and that there were no traces of an explosion. The discovery eliminates the possibility that a meteorite had struck the region.

‘Could it be linked to the global warming? Well, we have to continue our

research to answer this question,’ said Dr Plekhanov. ‘Two previous summers - years 2012 and 2013 were relatively hot for Yamal, perhaps this has somehow influenced the formation of the crater. ‘But we have to do our tests and research first and then say it more definitively.’

After the hole was discovered, there was speculation online about the crater indicating ‘the arrival of a UFO craft’. Ruling out extra-terrestrial intervention, Dr Plekhanov said: ‘We can say for sure that under the influence of internal processes there was an ejection in the permafrost. ‘I want to stress that was not an explosion, but an ejection, so there was no heat released as it happened.’