NATURALLY, I'M TRYING to figure out whether I should be scared. Cassell, who sports the brush cut and muscular physique of a soldier, isn't easy to read. He's modest and soft-spoken, despite an adrenaline-packed life. He grew up in Atlanta and started diving in nearby Lake Lanier at the age of 13. In 1980, when he was 18, he joined the Army to escape an alcoholic father. He learned to fly Aeroscout choppers with the Air Cavalry and became a Special Ops medic, a shadowy period he doesn't talk about except to say that he spent a lot of time in Central America on counter-drug operations.

Since leaving the military in 1996, Cassell has pieced together a living as a commercial diver and instructor, an underwater cameraman, and a security consultant (testing harbor defenses against the possibility of a terrorist attack). He estimates that he has spent more than 11,000 hours underwater, and over that time he's been attacked by a bull shark, pinned against the harbor bottom by a tanker, and treated like a chew toy by a territorial thousand-pound sea lion. So when he says stuff like "Humboldt squid are definitely capable of killing and eating a human being," it's hard to know whether the smile on his face is there because he loves swimming with danger or because he's having a great time trying to scare the shit out of me.

For perspective, I've turned to William Gilly, an avuncular 55-year-old biology professor

"Humboldts are definitely capable of killing and eating a human being," Cassell says.

at Stanford University's Hopkins Marine Station, in Pacific Grove, California. Gilly, a bluff, ruddy-cheeked man who prefers Hawaiian shirts to a lab coat, has been studying squid for 28 years and is one of only a handful of scientists who are real experts on the Humboldt. He starts with the anatomy basics, digging past a few pepperoni pizzas in a large freezer to pull out a dead Humboldt. It's just a few feet long, but it's a remarkable creature. It has large, black eyes and a sleek, rubbery body (called the mantle), which has two stabilizer fins at one end and tapers into a nest of eight arms and two longer tentacles at the other. Its arms are lined with suckers, which are ringed with small spiny teeth, and buried at the heart of the tentacular mass is a creepy-looking, articulating beak.

Gilly admits that very little is known about the squid or its life cycle. He and Cassell suspect it lives about two years. And they agree on something else: that the animals are spookily intelligent. They've both seen groups of Humboldt coordinating with one another to herd and attack schools of fish, and Cassell once saw one fiddling with the latch of an underwater cage he had just closed. "They have huge brains for their body size, much larger than most invertebrates," Gilly says. "They might be as smart as a dog."

The two men are equally in awe of the Humboldt's insatiable hunger, which drives the squid to eat just about anything it can catch, and ruthlessly cannibalize one another given the slightest opportunity. Cassell has seen a Humboldt trying to escape a pilot whale and snapping up little fish as it fled, a remarkable Darwinian undersea tableau. Gilly attributes this demonic appetite to simple need—in the course of a Humboldt's brief life it might grow from a fingertip-size squidling into a 100-pound-plus animal. As Gilly likes to note, that's roughly the equivalent of a human baby growing to the size of a blue whale—in just 24 months.

It is in the realm of squid-human relations that Gilly and Cassell part ways. Cassell is convinced that the squid are aggressive and potentially lethal to humans—he's been slammed and bruised enough times underwater, he says, and he believes at least one of the many stories of Mexican fishermen being attacked and killed. Gilly is not a diver, but he scoffs at the idea that the animals are dangerous predators and says that most Humboldt attacks take place when the squid are excited by the presence of food. "They are equipped to do damage, but so is a dog," he concludes.

This more benign view is based in part on Gilly's encounter with the squid during an expedition to the Sea of Cortez in 2002, when he jumped into the water wearing nothing but shorts, a T-shirt, and a mask. Within minutes he saw a group of five squid ascending from the depths, until they formed a perimeter around him. Then, one by one, they reached out and touched his outstretched hand. Gilly says he felt like he was meeting extraterrestrials coming by to say hello.

Gilly is reassuring, but it's hard to get past the cautionary tale of an underwater photographer named Alex Kerstitch, whose mugging by a group of squid is one of the founding cornerstones of the Humboldt-as-monster legend. One night in 1990, Kerstitch, a biologist at the University of Arizona, was diving in the Sea of Cortez with a film team. Suddenly a squid wrapped itself around his legs, and he felt himself being pulled backwards and down as others piled on. Kerstitch fought, but when the mauling was over, the Humboldt posse had ripped a gold chain from his neck and stripped him of his dive computer, light, and collection bag. Kerstitch died in March 2001, but California-based underwater cinematographer Bob Cranston was with him on the dive. "They really raped him," Cranston says. "Wherever his skin was exposed, like his neck, he had lots of abrasions."

Could Gilly be wrong? "I respect Gilly's work in the lab, but he's like a guy who has been on safari once and saw only lion cubs instead of big lions," Cassell says. "I'd love to take him diving and hover about ten feet away while he gets binged by some big ones. E.T., eh? Cute, eh?"

He's kidding. I think.