On June 30, 1908, something exploded in the atmosphere above the Tunguska region in Siberia. The blast left a concentric ring of flattened trees over an area of some 800 square miles.

The area of the epicenter of the explosion was so remote that it was some 20 years before an official scientific expedition was mounted, which found and documented the extensive devastation.

After World War II, Russian scientists compared the devastation found at Tunguska with that at Hiroshima, leading some to speculate that the Tunguska event was some sort of nuclear explosion.

Today, the best guess is that the event was the result of an encounter between the Earth and a comet or small asteroid which exploded high in the atmosphere. Although the area has been searched extensively by now, no impact crater or debris from the object itself has been found.

The Tunguska event serves as a critical lesson about the consequences of comet or asteroid impacts on Earth. Had the object entered the atmosphere and exploded above a major city, the devastation would be equivalent to that of an atomic bomb.

Consequently, scientists have devoted resources to identifying and tracking extra-terrestrial objects that might intersect the earth's orbit, and there is a good deal of discussion as to what should be done in the event that a threatening object is identified. There is also increasing evidence that such atmospheric explosions are relatively common, albeit on a much smaller scale.