On June 30, 1908, a giant fireball exploded over the forests of Tunguska in Siberia. The impact leveled hundreds of miles of forest, and the exact cause is a bit of a mystery. At the time of the explosion, it was too difficult to reach this remote part of Siberia to look for clues about what might have happened. But finally in 1927, a scientist named Leonid Kulik led the first research expedition to investigate the scene. However, they didn't find a crater or any meteorite fragments, and the mystery remained unsolved. Scientists now believe either a comet or an asteroid exploded in the atmosphere instead of striking Earth. This created an air burst with shock waves powerful enough to knock down a forest.