Except, unfortunately, what the heck it was. The Tunguska event was the mysterious explosion of unidentified origin that occured in a remote area of Siberia on 30 June 1908, flattening trees over 2000 square kilometers, but leaving no trace of a crater. Archy has put together a thorough account of what we know, including some of the speculation about the causes.
I rather liked the idea that it was a curse by the thunder god, Ogdy, mainly because “Ogdy” is such a cute name.