People living in southern Germany during Roman times may have witnessed a comet impact 5,000 times more destructive than the Hiroshima atom bomb, researchers say. Scientists believe a field of craters around Lake Chiemsee, in south-east Bavaria, was caused by fragments of a huge comet that broke up in the Earth’s atmosphere. Celtic artefacts found at the site, including a number of coins, appear to have been strongly heated on one side. This discovery, together with evidence from ancient tree rings and Roman reports of “stones falling from the sky”, has led researchers to conclude that the impact happened in...