An explosion occurs with a force 1000 times greater than the Hiroshima bomb. For hundreds of miles, blackened trees are slammed down in vast concentric rings, pointing away from the "ground zero" spot of incinerated earth. For days afterward the sky overhead becomes so bright that the evenings are illuminated well past sunset in towns a thousand miles away.

Is this an imaginary scenario describing the start of the third world war? No, this scenario is quite real, and in fact has already taken place - back in 1908. The blast occurred over a place called Tunguska in Siberia, and has remained one of the great scientific mysteries. A leading candidate, however, is an exploding meteor - one that blew with a force much greater than an atom bomb.

As a meteor passes into Earth's atmosphere, it experiences tremendous air resistance on its front end, but very little on its back end. It is this imbalance of forces that rip it apart; for a large meteor, hitting the earth's atmosphere can be just as destructive as hitting solid ground.

In fact, if the meteor explodes mid-air it may do more damage, because an air-explosion will set up a shock wave with a much wider dispersion. Scientists calculate that the Tunguska meteor blew about four miles above ground, which may also explain why no one has been able to find a crater or any remaining pieces.

This is only one possible explanation of what happened in Tunguska. If correct, however, it is a little reminder of how small even our most powerful weapons really are when compared to the forces unleashed in nature.