This theory has rapidly gained support from other scientists. Three years ago geologists found a submerged crater off the Yucatan coast of Mexico and identified it as the likely site of the impact. From the crater, scientists judged the asteroid was about 10 kilometers in diameter, with an energy release equivalent to 200 million hydrogen bombs.
A less devastating asteroid scored an Earth bullseye as recently as June 30, 1908. At least that's what scientists think happened near the Tunguska river in Siberia, when at 7:30 a.m. a huge flash lit the sky and an explosion, estimated as the equivalent of 800 Hiroshima bombs, flattened the Siberian forest for hundreds of square miles. Fortunately, the blast occurred in an unpopulated area, though it certainly startled the closest witness -- a woodcutter in front of his cabin 35 miles away. Recent analyses (by a team including Mac Low's collaborator Kevin Zahnle) conclude that the culprit was a stony asteroid about the diameter of a football field exploding five miles above the ground.
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