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Exactly 100 years ago today, on the morning of June 30, 1908, Russian villagers near the river Podkamennaya Tunguska in central Siberia reported a dark column of smoke and bright fiery flashes in a cloudless blue sky. Other eyewitnesses described tremors that damaged homes and powerful, hot winds that literally knocked people out of their chairs. The atmospheric effects of what was almost certainly an explosion estimated at about five miles (eight kilometers) above the remote Siberian taiga were noted as far away as London. Around Tunguska some 60 million trees over an area of 800 square miles (2,070 square kilometers) were scorched and leveled.

So what happened? Because there are no photographs of the blast, theories have abounded. Some scientists have suggested it was generated when chunks of antimatter made contact with Earth's atmosphere; others said interloping black holes. A few blamed malfunctioning alien spacecraft, likening the Tunguska event to a "Russian Roswell," in reference to the New Mexico town where conspiracy theorists claim a UFO crashed in 1947, only to be concealed by the U.S. government.

Decades after the event, however, scientists have pieced together enough evidence for a reasonable theory. The mysterious event's cause was not even investigated until 1927. The Tsarist Russian government at the time of the event did not look into the matter. In the intervening years it was distracted by World War I and toppled in the Russian Revolution of 1917, which also brought on the chaos of the Russian Civil War.

Finally, in 1927, Russian mineralogist Leonid Kulik led a Soviet expedition through the region. His pictures from this and other missions, along with subsequent field work done recently by a team from the University of Bologna in Italy to gather wood samples and sediment cores, suggest that a comet or a meteorite was the likely culprit.

One of the members of that team along with two colleagues, wrote about their findings in the June issue of Scientific American. Find out how close some other big meteorites have come to smacking Earth, and what is being done (or not done) to try and prevent future Tunguskas—or worse.

Click below to see a slide show of images from the site of the detonation over Siberia, as well as shots of meteorite impacts around the world.

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