You're running about a month behind on info. The answer to how it happened was already solved.http://www.dailygalaxy.com/my_weblog/2009/06/nasa-space-shuttle-solves-mystery-of-earths-greatest-explosion-the-1908-tunguska-event-in-siberia.html Abusive comment hidden. (Show it anyway.)
http://www.dailygalaxy.com/my_weblog/2009/06/nasa-space-shuttle-solves-mystery-of-earths-greatest-explosion-the-1908-tunguska-event-in-siberia.html