'End of the world' crater mystery
Scientists have been baffled by the discovery of a 260ft-wide hole in the ground in north Siberia.
It has the appearance of an 80m wide crater at the edges, but continues down into an underground chamber of unknown depth.
The sides of the crater are said to be fresh earth, which experts say indicates that it was created in the last few years, according to the Siberian Times.
A team of experts were launching an expedition to the site on Wednesday.
It was found by engineers who were flying past in a helicopter and filmed it, posting the footage on YouTube.
Its discovery on Russia's Yamal peninsula has prompted several theories as to how such a large hole could come to exist in such a remote place.
Theories have been fuelled by the name of the location it was found. Yamal means "end of the world" in the local language.
Experts say the most likely explanation is that it was created by an explosion of gas that had built up in a naturally occurring chamber under the surface.
The ring of fresh earth surrounding the hole gives the impression of a mass of material that has burst out from below, they believe.
Global warming has been blamed by some theorists who say that the boggy area - which had previously been permanently frozen - has thawed and begun releasing hydrocarbon gases previously trapped underneath the tundra.
One of the less likely explanations is that it was caused by a UFO.
The Yamal branch of Russia's Emergencies Ministry told the Siberian Times it was too early to say what caused the gigantic hole in the earth.
A spokesman told the paper: "We can definitely say that it is not a meteorite. No details yet."
An expedition has been organised to visit the site in order to understand how it could have come to exist.
The authorities are keen to work out what caused the hole as the area is used to extract natural gas and the Siberian Times says there are fears that changes in the landscape could make drilling more dangerous.