A century ago in Siberia

One hundred years ago today something fell from the sky and flattened hundreds of square miles of forest in a remote part of Siberia. Called the Tunguska event (after a nearby river), it was an explosion estimated to be a thousand times larger than the nuclear blast that leveled Hiroshima in World War II. It was so incredibly massive that the sound of the explosion was heard hundreds of miles away, it threw enough debris into the atmosphere to reflect enough sunlight that the night sky in London - thousands of miles away - was bright enough to read a newspaper.

Still, 100 years later scientists still aren't quiet sure what caused Tunguska. Some of the wilder theories say the blast was caused by a mini black hole, or perhaps an exploding alien space ship. The most popular theory though is that the blast was caused by a rocky asteroid about 100 feet across.

Tunguska is so remote that scientists didn't reach the area for nearly 20 years after the event, due in part to the fear the local Evenk people had of the impact area – they regarded it as a place of evil. Scientists were also confused by the lack of a crater, since asteroid impacts typically make craters (just look at the surface of the moon). The main theory is now that the asteroid blew up several miles above the surface. Strangely the hundreds of square miles of trees knocked down in the Tunguska event didn't fall in a circle, but instead in a pattern called the "Tunguska butterfly" because it resembles, well, a butterfly. This same pattern was also seen in the debris of Hiroshima (the atomic bomb there exploded not on impact with the ground, but about a mile above it).

Scientists estimate a Tunguska-sized asteroid probably hits the Earth every 300 years, so hopefully we'll be safe for another couple of centuries.