Near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai in Russia, a large meteoroid or comet explodes at five to ten kilometers above the Earth’s surface with a force of about twenty megatons of TNT. The explosion is a thousand times more powerful than the Hiroshima atomic blast, knocks over an estimated 80 million trees and generates a shock wave estimated to have been 5.0 on the Richter scale.