The crater has an icy lake at its bottom with water cascading down its eroding permafrost walls. Picture: Andrey Naumenko, 'Yamal-Region'

The crater on the Yamal Peninsula was caused by aliens, a meteorite, a stray missile, or an explosive gas cocktail released due to global warming, according to various theories in recent days.

Images of the remarkable phenomenon have gone round the world since The Siberian Times highlighted helicopter images of the giant hole earlier this week.

The first expedition to the scene - the scientists have just returned - took these epic pictures of the hole, including the darkening pattern on the inner rim.

Now they are using Russian satellite pictures to fix the moment when it suddenly formed.

They found the crater - around up to 70 metres deep - has an icy lake at its bottom, and water is cascading down its eroding permafrost walls.

It is not as wide as aerial estimates which suggested between 50 and 100 metres.

The first scientific expedition has just returned from the site, now the experts are working with satellite pictures to fix the moment when the crater has formed. Pictures: Marya Zulinova, press service of the Governor YaNAO

While the team insist that it is a 'natural phenomenon', they insist further study is essential to understanding the formation of the crater in lake-studded Yamal - a name which means 'end of the world' in the local tongue.

Andrey Plekhanov, Senior Researcher at the State Scientific Centre of Arctic Research, said: 'The crater has more of an oval than a circular shape, it makes it harder to calculate the exact diameter. As of now our estimates is about thirty metres.

'If we try to measure diameter together with soil emission, the so-called parapet, then the diameter is up to sixty meters.

'The crater is from 50 to 70 metres deep'.

The first look down the crater. Picture: Andrey Naumenko, 'Yamal-Region'

The researchers were unable to make their way to the bottom of the lake, but did go inside the crater.

'There is ice inside the crater which gradually thaws under the sun.

'Also there is melted water flowing down from its sides, you can see water traces on the pictures. The crater is filled with ice by about eighty per cent. '

He stressed: 'We are working with space photographs to figure out exact time of its formation.

'We have taken soil and ice samples which went straight to laboratories. We can be certain in saying that the crater appeared relatively recently, perhaps a year or two ago; so it is a recent formation, we are not talking about dozen years ago.

'Could it be linked to the global warming? We have to continue our research to answer this question.

'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'.

The crater is filled with ice by about eighty per cent. Pictures: Marya Zulinova, press service of the Governor YaNAO

The best theory for now is that the crater was formed by internal - not external - forces.

'For now we can say for sure that under the influence of internal processes there was an ejection in the permafrost. I want to stress that it was not an explosion, but an ejection, so there was no heat released as it happened'.

Earlier scientists were sure there was burning visible on the sides of the crater.

'I also want to recall a theory that our scientists worked on in the 1980s - it has been left and then forgotten for a number of years.

'The theory was that the number of Yamal lakes formed because of exactly such natural process happening in the permafrost.

'Such kind of processes were taking place about 8,000 years ago. Perhaps they are repeating nowadays. If this theory is confirmed, we can say that we have witnessed a unique natural process that formed the unusual landscape of Yamal peninsula.

Experts examine the newly-discovered hole in the earth in Siberia. Pictures: Marya Zulinova, press service of the Governor YaNAO

'There was no traces of anthropogenic impact near the crater, just as there was no traces of human presence, except for very few sledge traces and of course reindeer traces.

'If it was a man-made disaster linked by gas pumping, it would have happened closer to the gas fields', Andrey Plekhanov told The Siberian Times.

These are about 30 kilometres away. 'Gas workers would have been on alert, letting us know about it immediately.'

Denigrating speculation of aliens or UFOs he insisted:'There is nothing mysterious about it. There is no weird or unexplained feelings there, we came back safe and sound'.

Despite this, he said: 'I've never seen anything like this, even though I have been to Yamal many times.'

The hole is in the Yamalo-Nenets Autonomous, some 30 kilometres from the Bovanenkovo gas field. Pictures: Marya Zulinova, press service of the Governor YaNAO

The crater is different from others on Yamal.

'There is nothing mysterious here, it is simply Mother Nature's law with its internal pressure and changes in temperatures', Andrey Plekhanov said.

Marina Leibman,Senior Researcher at the Earth's Cryosphere Institute of the Siberian Branch of the Russian Academy of Sciences, said: 'About the future of the crater - its walls are constantly thawing, water is gathering up and I suspect that it gets frozen at the bottom of the crater. If the water stream intensifies - for example because of the hot second part of July - then it won't have enough time to freeze. This will likely lead to a formation of a new lake.'

Vladimir Pushkarev, Director of the Russian State Scientific Center of the Arctic Research, 'It is an interesting phenomenon, there is every sense in continuing scientific work on it and right now we are discussing the best ways of exploring the site.'

The hole is in the Yamalo-Nenets Autonomous, some 30 kilometres from the Bovanenkovo gas field.

Anna Kurchatova from the Sub-Arctic Scientific Research Centre thinks the crater was formed by a water, salt and gas mixture igniting an underground explosion, the result of global warming.

Gas accumulated in ice mixed with sand beneath the surface, and that this was mixed with salt - some 10,000 years ago this area was a sea.

Global warming, causing an 'alarming' melt in the under soil ice, released gas causing an effect like the popping of a Champagne bottle cork, she suggests.

Yamal, a large peninsula jutting into Arctic waters, is Russia's main production area for gas supplied to Europe.