On June 30, 1908, a violent explosion occurred in the atmosphere over the Podkamennaya Tunguska River in a remote part of central Siberia. The blast’s consequences were similar to a hydrogen bomb going off, leveling thousands of square miles of forest. The shock of the explosion was heard more than 600 miles (960 kilometers) away. A number of theories have been proposed to account for this event.

Some people thought that a large meteorite or a piece of antimatter had fallen to Earth. But a meteorite, composed of rock and metal, would have created a crater and none was found at the impact site. There are no high radiation levels in the area that would have resulted from the collision of antimatter and matter. Two other theories include a mini-black hole striking Earth or the crash of an extraterrestrial spaceship. However, a mini-black hole would have passed through Earth and there is no record of a corresponding explosion on the other side of the world. As for the spaceship, no wreckage of such a craft was ever found.

The most likely cause of the explosion was the entry into the atmosphere of a piece of a comet, which would have produced a large fireball and blast wave. Since a comet is composed primarily of ice, the fragment would have melted during its passage through Earth’s atmosphere, leaving no impact crater and no debris. Since the Tunguska Event coincided with Earth’s passage through the orbit of Comet Encke, the explosion could have been caused by a piece of that comet.