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Magill says the squid came out of nowhere.
"I was pulled from behind, he grabbed my inflator hose and my light ... And pulled me backwards and started pulling me down," said Magill. "I couldn't tell which way was up or down."
The ferocious attack off the coast of San Diego came as no surprise to longtime diver and Humboldt squid aficionado Scott Cassell.
"I've been attacked thousands of times by Humboldt squid," said Cassell. "They just come out of the dark and they're on you instantly."
Unlike most divers, Cassell seeks out the Humboldt squid in hopes of learning more about the creatures Mexican fishermen call "red devils."
"I've had my right arm yanked out of its socket by Humboldt squid. I've been dragged down so quickly, that my right ear drum ruptured," said Cassell.
Cassell now uses armor when he dives with the squid. It keeps him alive, but doesn't lessen the impact.
Humboldt squid are native to the deep waters off Mexico. But now, they're swarming off the Southern California coast.
"Humboldt squid can have up to 20 million babies a piece. And so, we literally have a population explosion off the coast of California," said Cassell.
Experts say one reason for the sudden swarm is the over-fishing of the ocean.
"Large apex predators like sharks, like billfish and marlin, that were once very common in these areas ... now that they've been overfished, we're seeing a lot more squid," said Steve Blair, Aquarium of the Pacific.
Squid feed on salmon. The high population of ravenous squid has contributed to the collapse of the $1.4 billion salmon fishery off the California coast.
"There was approximately an 80 percent decline in salmon almost overnight," said Blair.
The squid can be dangerous, even for professionals. Seconds after a picture was taken of Cassell with a seven-foot squid, it attacked him.
"When he ripped free, he came back and attacked me on the head, and lacerated my scalp so bad it exposed my skull," said Cassell.
Despite the danger, Cassell has devoted much of his life to studying the Humboldt squid.
"They have ice blue blood, they have three hearts, [and] the color of their skin changes in the blink of an eye," said Cassell, who says the squid also have a spike-covered tongue and 1,200 suction cups lined with 36 teeth.
Each squid has a total of about 35,000 teeth.
Although the squid clash with sharks and other sea creatures, Cassell warns Southern California divers are at risk.
"They will kill and eat a human being. I've documented the human attacks in the Sea of Cortez and I've interviewed fishermen who have watched other fishermen die," said Cassell.
Shanda Magill was back in the water three days after her attack, but still holds a bit of a grudge.
"Four or five days later, I went out with friends. I had calamari," said Magill. "It was delicious."