A unique "double crater" created by two massive meteorite impacts 460 million years ago has been discovered in Sweden – the first time a strike of this kind has ever been found on Earth. The twin strike created the crater in Jämtland, with the larger measuring 7.5km in diameter and the smaller measuring 700m.

Scientists from the University of Gothenburg said the strikes took place during an era of meteorites, when a collision between two large asteroids led many fragments to come raining down on Earth. Geophysicist Erik Sturkell said: "The two meteorite impacts occurred at the same time, 458 million years ago, and formed these two craters.

"Around 470 million years ago, two large asteroids collided in the asteroid belt between Mars and Jupiter, and many fragments were thrown off in new orbits. Many of these crashed on Earth, such as these two in Jämtland."

At the time, Jämtland was under the sea, submerged at a depth of around 500m, when the meteorites struck simultaneously. The scientists said double impacts of this kind are extremely unusual, with this being the first to ever be conclusively proved on Earth. Sturkell explained: "Information from drilling operations demonstrates that identical sequences are present in the two craters, and the sediment above the impact sequences is of the same age. In other words, these are simultaneous impacts."

The impact forced the water away, and for 100 seconds the massive craters were completely dry, they said. "The water then rushed back in, bringing with it fragments from the meteorites mixed with material that had been ejected during the explosion and with the gigantic wave that tore away parts of the sea bed," Sturkell added.