At 7:17am on 30 June 1908, an immense explosion tore through the forest of central Siberia. Some 80 million trees were flattened over an area of 2,000 square km (800 square miles) near the Tunguska River.
The blast was 1,000 times more powerful than the atomic bombs dropped on Hiroshima and Nagasaki and generated a shock wave that knocked people to the ground 60km from the epicentre.
The cause was an asteroid or comet just a few tens of metres across which detonated 5-10km above the ground, 100 years ago today...
http://news.bbc.co.uk/2/hi/science/nature/7470283.stm