The Tunguska event was an enormously powerful explosion that occurred near (and later struck) the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, on June 30, 1908. The explosion is believed to have been caused by the air burst of a large meteoroid or comet fragment at an altitude of 5–10 kilometres (3–6 mi) above the Earth’s surface.
Although the meteoroid or comet appears to have burst in the air rather than hitting the surface, this event still is referred to as an impact. Estimates of the energy of the blast range from 5 to as high as 30 megatons of TNT, with 10–15 megatons of TNT the most likely—about 1,000 times more powerful than the atomic bomb dropped on Hiroshima, Japan.
The Tunguska explosion knocked an estimated 80 million trees down over an area covering 2,150 square kilometres (830 sq mi) (i.e. circular area of 52km in diameter). It is estimated that the shock wave from the blast would have measured 5.0 on the Richter scale.