As reported by Ars Technica:

Simon Redfern wrote:

On the morning of June 30 in 1908, a gigantic fireball devastated hundreds of square kilometres of uninhabited Siberian forest around the Tunguska river. The first scientists to investigate the impact site expected to find a meteorite, but they found nothing. Because no traces of a meteorite were found, it many scientists concluded that the culprit was a comet. Comets, which are essentially muddy ice balls, could cause such a devastation and leave no trace.

But now, 105 years later, scientists have revealed that the Tunguska devastation was indeed caused by a meteorite. A group of Ukrainian, German, and American scientists have identified its microscopic remains. Why it took them so many years makes for a fascinating tale about the limits of science and how we are pushing them.

Excellent read. I can only hope this puts one more alien conspiracy theory to rest.