The Tunguska event was an enormously powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, at about 07:14 local time 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 3–6 miles above the Earth's surface. Different studies have yielded widely varying estimates of the object's size. It is the largest impact event on or near Earth in recorded history.
The explosion registered on seismic stations across Eurasia. In some places the shock wave would have been equivalent to an earthquake of 5.0 on the Richter scale. It also produced fluctuations in atmospheric pressure strong enough to be detected in Great Britain. Over the next few days, night skies in Asia and Europe were aglow; it has been theorized that this was due to light passing through high-altitude ice particles formed at extremely low temperatures, a phenomenon that occurred again when the Space Shuttle re-entered the Earth's atmosphere. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months, from suspended dust.