A full century after the mysterious Tunguska explosion in Siberia leveled an area nearly the size of Tokyo, debate continues over what caused it.
Many questions remain as to what crashed into the Earth from above — how big it was and what it was made of. Some question whether it even came from space at all, or whether it erupted from the ground instead.
And there is always speculation that it was caused by a UFO or famed inventor Nikola Tesla's "death ray."
The explosion near the Podkamennaya Tunguska River on June 30, 1908, flattened some 500,000 acres (2,000 square kilometers) of Siberian forest. Scientists calculated the Tunguska explosion could have been roughly as strong as 10 megatons to 20 megatons of TNT — 1,000 times more powerful than the atom bomb dropped on Hiroshima.
The longstanding theory regarding the cause of the event is a cosmic impact from an asteroid or comet. In the last decade, researchers have conjectured the event was triggered by an asteroid exploding in Earth's atmosphere and measuring roughly 100 feet wide (30 meters) and 617,300 tons (560,000 metric tons) in mass — more than 10 times that of the Titanic.
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100 years later, Tunguska remains mysterious