On the sea bottom off the coast of central California lies a vast field of mysterious pockmarks. The field spans about 500 square miles (1,300 square kilometers), and there are approximately 5,200 of these strange pits, which on average measure about 600 feet in diameter (175 meters) and 16 feet (5 m) deep.

No one knows how these mystery holes appeared, but they likely made their first appearance 400,000 years ago, according to research presented on Dec. 9 at the annual meeting of the American Geophysical Union (AGU) here by Charles Paull, a marine geologist and senior scientist with the Monterey Bay Aquarium Research Institute (MBARI).

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