What is the Tunguska event?

On June 30, 1908 deep in the remote recesses of central Siberia, Russia, and 50-100 meter (150-330 feet) asteroid exploded into the atmosphere over the Tunguska region. The ensuing blast yield- estimated to be 3-5 megatons TNT -  was the largest ever recorded by humans. Some estimates place the blast yield up to 30 megatons TNT. Despite the fact that the asteroid was almost completely destroyed as it disintegrated into the atmosphere, about 5-10 kilometers (3-6 miles) above the Earth's surface, the blast wave was powerful enough to decimate over 2000 km2 (~800 square miles) of taiga forest. Fortunatley, because of the remote, sparsely populated region, no one is beleived to have bee killed by the blast. The first research expedition to the site in 1927 failed to find any meteorite fragment at the site leading to speculation that the blast was perhaps the result of a comet impact (a comet whose ice might have rapidly dissipated into the atmosphere but still produce the destructive blast wave). However, later expedtions did identify tiny spherules with high nickel to iron concentrations, consistent with metals originating from cosmic, not terrestrial origins. An impact event on the scale of the Tunguska impact occurs on average every 200-300 years. More info on NASA's The Tunguska Impact--100 Years Later.

Above: Flattened trees of the Siberian taiga in the Tunguska region. Photo Leonid Kulik, 1927.