A large meteor crashes through the Earth's atmosphere and explodes above the surface, leaving no crater. A similar event is thought to have happened in Tunguska in 1908. However, in this case, while the explosion occurred above the ground, the shock front from the blast penetrated to ground level and deposited tiny grains from the original meteor, melted and fused together. Researchers found these grains in Antarctica and dated them to 430,000 years ago. They believe this is the first ever detection of a 'touchdown meteor'.