Tunguska, Russia, somewhere in Siberia. Importance: June 30, 1908, before the advent of the atom bomb mind you, an explosion about the size of 1000 Hiroshima bombs occured above the earth's surface. Favorite theories: small chunk of antimater, tiny black hole passing through the earth (though would have had same effect on other side), or most commonly held theory that a comet hit the atmosphere and blew up before actually hitting the earth. Russians have a tank named after it.