On June 30, 1908, a rock estimated to about 100 meters (328 feet) in diameter exploded above the Tunguska region of Siberia, which flattened trees over 2000 square kilometers (800 square miles) and produced a shock wave that knocked people to the ground at a distance of tens of kilometers (tens of miles) from the detonation point. This image is from the Leonid Kulik expedition in 1927.