On June 30, 1908, a fireball streaked through the Siberian sky, followed by an enormous explosion that leveled 770 square miles (2,000 square kilometers) of remote forest. Scientists later calculated that the Tunguska event, named after a nearby river, released an amount of energy 1,000 times greater than the atomic bomb dropped on Hiroshima in 1945 [source: Valsecchi].
It took researchers until 1927 to reach the remote site, and the inability to find a clear-cut impact crater or pieces of a meteor led to some fantastic theories, including scenarios involving anti-matter and UFOs. Others suspected that Earth had been struck by a comet —-– since a comet is basically a ball of ice, it wouldn't have left a trace. But in 2007, a team of Italian researchers used acoustic imaging to identify the crater, which turned out to be in a lake 5 miles (8 kilometers) north of the spot originally identified by scientists [source: Valsecchi].
Ukrainian researchers then proved that Tunguska was, in fact, caused by a meteor, according to a 2013 article in Planetary and Space Science. They analyzed samples of peat dating from that summer, and found they contained fragments of minerals found in meteorites, as well as lonsdaleite, a substance known to form from shock waves following an explosion. Just as significant, the combination of all these elements was nearly identical to a meteor impact site in Arizona [source: Redfern].