The Tunguska event, or Tunguska blast or Tunguska explosion, was an enormously powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, at about 7:14 a.m. KRAT (0:14 UT) on June 30 [O.S. June 17, 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. Different studies have yielded varying estimates of the object's size, with general agreement that it was a few tens of metres across. The number of scholarly publications on the problem of the Tunguska explosion since 1908 may be estimated at about 1,000 (mainly in Russian). Many scientists have participated in Tunguska studies, the best-known of them being Leonid Kulik, Yevgeny Krinov, Kirill Florensky, Nikolai Vladimirovich Vasiliev and Wilhelm Fast. Although the meteoroid or comet burst in the air rather than hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 to as high as 30 megatons of TNT (21--130 PJ), with 10--15 megatons of TNT (42--63 PJ) the most likely—roughly equal to the United States' Castle Bravo thermonuclear bomb tested on March 1, 1954, about 1,000 times more powerful than the atomic bomb dropped on Hiroshima, Japan, and about one-third the power of the Tsar Bomba, the largest nuclear weapon ever detonated. The explosion knocked over an estimated 80 million trees covering 2,150 square kilometres (830 sq mi). It is estimated that the shock wave from the blast would have measured 5.0 on the Richter scale. An explosion of this magnitude is capable of destroying a large metropolitan area. This possibility has helped to spark discussion of asteroid deflection strategies. The Tunguska event is the largest impact event over land in Earth's recent history. Impacts of similar size over remote ocean areas would most likely have gone unnoticed before the advent of global satellite monitoring in the 1960s and 1970s.