On June 30, 1908, at 7:14 am in central Siberia, local farmer Semen Semenov witnessed an extraordinary event: he described seeing "the sky split in two," with fire appearing high and wide over the forest. From the fire's direction came intense heat. This was followed by the sky seemingly closing in and a powerful thump that threw Semenov a few yards away. The noise that followed was akin to cannons firing and the earth shook beneath him. Semenov, who witnessed the Tunguska event, the largest cosmic impact in modern history, observed a fiery explosion 65 kilometers (40 miles) from ground zero. The blast's effects extended into Northern Europe and Central Asia.
Since the Tunguska event, both scientists and enthusiasts have speculated about its cause. The mystery remains unsolved. Care to put the pieces together? Click through the gallery to learn more.