Tunguska
Hi, all - I've been away for a couple of weeks, so my apologies if this has been covered already.
Andrei Yu. Ol'khovatov has put forward an explanation of the Tunguska event of 1908 that doesn't involve comets, asteroids or any space-originating cause. He addresses the many problems with the impact theories and presents a strong case for it being a "tectono-atmospheric phenomenon".
http://www.geocities.com/CapeCanaveral/Cockpit/3240/tunguska.htm
Some interesting info on "Barisal Guns" and "brontides" - mysterious loud noises heard in many parts of the world.
http://www.lewis-clark.org/boom/moodus_noises.htm
If correct, this explanation would somewhat weaken the NEO impact threat, I suppose.