What caused the Tunguska explosion?

The force of the explosion was equivalent to twenty million tonnes of TNT and a thousand times greater than that of the atomic bomb dropped on Hiroshima in 1945. An estimated sixty million trees were felled over an area of over two thousand square kilometres and yet apparently nobody was killed.

It happened at 7.15 am on 30th June 1908 when a giant fireball, as bright the sun, exploded in the sky over Tunguska in central Siberia. The suggested explanations vary from asteroids and comets to ball lightning, black holes and alien spaceships.

Next year will see the centenary of this extraordinary event and no doubt there will be all sorts of articles and programmes about it. This evening, I watched a fascinating programme on the History Channel which will be repeated. Also you can find further information on the event here.