Scientists believe they may finally have found a crater produced by one of the biggest cosmic impacts with Earth in recent history.

In 1908, a small comet or meteor exploded over Tunguska, a remote region of Siberia, flattening trees for up to 2,000 square kilometers.

The blast is estimated to have been equivalent ten to 15 million tons of TNT. There were a few dramatic eyewitness accounts of a huge fireball and bright glows were seen over Europe and Asia. Despite this, the region was so remote that no expeditions were sent until the late 1920s.

However, no traces of the impacting body have ever been found, and investigators concluded that the cosmic missile exploded 5-10 km above the ground.

Now an Italian team has examined a 300-metre wide lake just 8 km NNW of the centre of the explosion and have concluded that it probably fills a crater formed in a secondary impact following the initial blast. Due to the remoteness of the region, no one is able to say that the lake was there before the Tunguska Event – it does not feature on any maps.

The scientists base their conclusions on the funnel-like form of the lake’s bottom plus imaging of sedimentary deposits within it. The team, who publish their findings on the online site of the journal Terra Nova, say the object struck swampy ground covered in permafrost and this affected the size and shape of the crater.

They say the bowl-shape matches that of similar impact craters on Earth, and is slightly elliptical. They suggest the impactor was a fragment left from the iniatial blast and was up to ten metres across. They also believe that it struck the Earth at a low velocity and so is likely still to be buried beneath the lake.

A much bigger explosion of a comet over North America may have wiped out Stone-Age tribes, mammoths and other creatures nearly 13,000 years ago. You can read the new Tunguska paper in full here.

The photo is from an expedition to Tunguska in 1927, and shows trees flattened by the explosion 19 years earlier.

Footnote: UK expert Dr Gareth Collins, of Imperial College London, has said he is skeptical about the Italian claims. He told BBC Online: “In my opinion, they certainly haven’t provided any conclusive evidence it’s an impact structure.

“The impact cratering community does not accept structures as craters unless there is evidence of high temperatures and high pressures. That requires evidence of rocks that have been melted or rocks that have been ground up by the impact.”

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