Geologists prospecting for oil in the Barents Sea have stumbled across the largest meteorite crater ever found in Europe. It is also one of the largest in the world.

It was formed 150 million years ago when an asteroid, possibly 500m (550 yards) across and travelling at 30,000 km/h (19,000 mph), plunged into the sea off the coast of Norway.

The impact site

It would have caused worldwide devastation resulting in global climate change and the extinction of many species.

At the site of the impact there would have been a mushroom cloud of superheated steam. Temperatures of over 10,000 degrees centigrade would have melted many tonnes of rock. Gigantic tidal waves would then have raced around the world from Canada to Russia.

After the initial fury, dust and other particles thrown into the atmosphere would have created a cloud that blocked out the sunlight starting a "nuclear winter".

Many species not wiped out by the initial impact would have died out during the prolonged cold and lack of sunlight in this extended winter.

Accidental discovery

The discovery of the crater in the Barents Sea was accidental, following a search for potential oil and gas reservoirs.

At first geologists thought it was an ordinary salt formation or a submarine volcano. But Steinar Gudlauggson, from the Department of Geology at the University of Oslo, had suspicions that it was an impact crater.

Geologists looked at the data more closely and concluded that it could be a parallel to the Chicxulub Crater in Mexico, the imprint left by the meteorite which caused the extinction of the dinosaurs.

Intense deformation

The proof came after the examination of rock samples drilled from close to the crater. They studied 400,000 quartz grains and found they had been deformed by an intense, sudden shock wave.

They also found traces of the rare element iridium which is far more common in objects from space than on the Earth's surface.

One of the drilled rock cores, 121 m (130 yards) long, has been described as a geological gem. It is one of the few cases where both the crater and the dust and rock blasted out by the impact have been found and collected. The crater rock and the debris on top therefore carry unique information about the impact.

Although the Earth has suffered a steady bombardment from space over geological time, only 160 impact craters have been identified. The Mj�lnir crater is only the seventh marine meteor crater found.