The Tunguska event is a massive explosion that occurred on the morning of June 30, 1908 at around 07:17 local time in a remote region approximately 750 km northwest of Lake Baikal. Natives and Russian settlers in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the sun moving across the sky. About ten minutes later, there was a sound similar to artillery fire. The sounds were accompanied by a shock wave that knocked people off their feet, broke windows and some reports suggest that at least three people may have died in the event. It also flattened an estimated 80 million trees over an area of 2,150 km2 (830 sq mi) of forest. The explosion is generally attributed to the air burst (not impact itself) of a meteoroid about 100 meters in diameter that exploded at an altitude of 5 to 10 kilometres (3 to 6 miles). Due to the remoteness of the area, only a few expeditions reached the site. One of them was organized by the University of Bologna in 1991, when this wood plate was taken.