The Tunguska event was a large explosion that occurred near the Stony Tunguska River, in what is now Krasnoyarsk Krai, Russia, on the morning of June 30, 1908 (N.S.). The explosion over the sparsely populated Eastern Siberian Taiga flattened 2,000 km2 (770 sq mi) of forest and caused no known casualties. The cause of the explosion is generally thought to have been a meteor. It is classified as an impact event, even though no impact crater has been found; the meteor is thought to have burst in mid-air at an altitude of 5 to 10 kilometres (3 to 6 miles) rather than hit the surface of the Earth. Different studies have yielded varying estimates of the superbolide's size, on the order of 60 to 190 metres (197 to 623 feet), depending on whether the meteor was a comet or a denser asteroid. It is considered the largest impact event on Earth in recorded history.Since the 1908 event, there have been an estimated 1,000 scholarly papers (mainly in Russian) published on the Tunguska explosion. Many scientists have participated in Tunguska studies: the best known are Leonid Kulik, Yevgeny Krinov, Kirill Florensky, Nikolai Vladimirovich Vasiliev, and Wilhelm Fast. In 2013, a team of researchers led by Victor Kvasnytsya of the National Academy of Sciences of Ukraine published analysis results of micro-samples from a peat bog near the center of the affected area showing fragments that may be of meteoritic origin.Estimates of the energy of the air burst range from 30 megatons of TNT (130 PJ) to 10 and 15 megatons of TNT (42 and 63 PJ), depending on the exact height of burst estimated when the scaling-laws from the effects of nuclear weapons are employed. While more modern supercomputer calculations that include the effect of the object's momentum estimate that the airburst had an energy range from 3 to 5 megatons of TNT (13 to 21 PJ), and that simply more of this energy was focused downward than would be the case from a nuclear explosion.Using the 15 megaton nuclear explosion derived estimate is an energy about 1,000 times greater than that of the atomic bomb dropped on Hiroshima, Japan; roughly equal to that of the United States' Castle Bravo ground-based thermonuclear test detonation on March 1, 1954; and about two-fifths that of the Soviet Union's later Tsar Bomba (the largest nuclear weapon ever detonated).It is estimated that the Tunguska explosion knocked down some 80 million trees over an area of 2,150 square kilometres (830 sq mi), and that the shock wave from the blast would have measured 5.0 on the Richter scale. An explosion of this magnitude would be capable of destroying a large metropolitan area, but due to the remoteness of the location, no fatalities were documented. This event has helped to spark discussion of asteroid impact avoidance.