Tunguska event (tung-guss -kă) A gigantic explosion that occurred at about 7.17 a.m. on June 30, 1908 in the basin of the River Podkamennaya in Tunguska, Central Siberia. Devastation rained over an area 80 km in diameter and eye witnesses up to 500 km away saw in a cloudless sky the flight and explosion of a blindingly bright pale blue bolide. The sound of the explosion reverberated thousands of kilometers away, the explosion air wave recorded on microbarographs going twice round the world. The main explosion had an energy of 5 × 1016 joules and occurred at an altitude of 8.5 km. It was caused by the disintegration of an incoming object, most likely a Fragile Apollo asteroid or a small comet nucleus. When the object encountered the Earth it would have been coming from a point in the dawn sky comparatively close to the Sun and would thus have been most difficult to detect and observe.