It's a deep-sea mystery worthy of a Jules Verne novel.

Last month thousands of large squid mysteriously beached themselves on California shores.

It was not the first time such a mass "suicide" has occurred. Observers say it seems to happen every few years.

But what causes the Humboldt—or jumbo—squid (Dosidicus gigas) to end up on land?

"We don't know," said William Gilly, a biologist at Stanford University's Hopkins Marine Station in Pacific Grove, California. "It's a mystery of large size what's killing these squid."

Gilly has studied squid for more than two decades and has been tagging the jumbo squid in Mexico's Gulf of California as part of a larger study of their movements in the Pacific Ocean. He speculates that the cause of the recent deaths might be a combination of the squid spending too much time in warm water and the squid eating something toxic.

Red Devil

The elusive Humboldt squid can reach 6 feet (1.8 meters) long. It has powerful arms and tentacles, excellent underwater vision, and a razor-sharp beak that easily tears through the flesh of its prey. The squid's fearsome reputation has earned it the nickname "red devil."

But very little is known about its biology, and most of its habits are a mystery to scientists. The animals do not survive for more than a few days in captivity, and studying their behavior in the field is hard without interfering with them.

"We know so little about them because they spend 95 percent of their lives at depths well beyond those safely observed with scuba," Gilly said. "We don't know where they spawn, and their eggs have never been found in the wild."

Scientists believe the squid live at depths of 660 to 2,300 feet (200 to 700 meters) during the day. Their preferred depth at night is about 220 feet (70 meters).

Although elusive, the squid are not rare. Gilly estimates that up to 10 million squid may be living in a 25-square-mile (65-square-kilometer) area outside Santa Rosalia, Mexico, where they are the target of a major fishery.

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SOURCES AND RELATED WEB SITES Ocean Institute Hopkins Marine Station of Stanford University Santa Barbara Museum of Natural History