One hundred years ago today, St. Petersburg, Russia, would have been annihilated by an enormous explosion—if the detonation had occurred less than five hours later than it actually did. But as it turned out, the event happened thousands of kilometres further east, smack dab in the middle of Siberia. St. Petersburg was lucky, saved by the rotation of the earth.
The fireball now known as the Tunguska Event was the mid-air explosion of an extraterrestrial object of some sort (link via PZ Myers), likely a meteorite or fragment of a comet—fairly big, perhaps 50 or 60 metres in diameter, like a condominium tower falling from the sky at several thousand kilometres per hour—which disintegrated violently in the atmosphere. The place is still a remote Siberian forest, several hundred kilometres northwest of Lake Baikal. Trees that fell then still lie on the ground a century later. The heat created microscopic glass beads in the soil. There would be no airburst of similar magnitude until the invention of the hydrogen bomb.
It took almost 20 years for a Soviet scientific expedition, led by Leonid Kulik, to visit the area and survey the still-extensive damage in 1927: while there was no obvious crater, trees had been stripped of their branches and bark, as well as flattened by the shock wave, in a region some 50 km wide. Had the Tunguska Event occurred in a populated area, tens or hundreds of thousands of people might have died.
It was nothing, of course, compared to other impact events on earth, such as those linked with mass extinctions in the more distant past. But I still wouldn't have wanted to be anywhere near it.
Incidentally, Tunguska shares its centenary not only with my 39th birthday today, but also with something else I'd probably prefer to steer clear of: the 45th birthday of Yngwie Malmsteen.