Fifteen hours of driving across the sun-blasted beauty of the Baja Peninsula has transported us from the hustle of San Diego to the dusty Mexican town of Santa Rosalía, tucked into the brown hills along the western edge of the Sea of Cortez.

These are the evocative waters that inspired John Steinbeck to muse in his Log from the Sea of Cortez that "men really need sea-monsters in their personal oceans." I'm here to dive into Cassell's personal ocean and to meet his favorite sea monster. Up high, in the warm sun and soothing breeze, it seems like a good idea. Except that there are a couple of unsettling questions. Like: How often is a monster of myth a monster in reality? And am I going to get the crap knocked out of me 50 feet down?

I worry about these things because Cassell, 44, a world-class diver, underwater cameraman, and Special Operations vet from Escondido, California, is out to convince me—live and up close—that the undersea world's most intriguing predator is not one of the usual suspects (like the great white shark or killer whale) but a powerful, outsize squid that features eight snakelike arms lined with suckers full of nasty little teeth, a razor-sharp beak that can rapidly rip flesh into bite-size chunks, and an unrelenting hunger. It's called the Humboldt, or jumbo, squid, and it's not the sort of calamari you're used to forking off your dinner plate. This squid grows to seven feet or more and perhaps a couple hundred pounds. It has a rep as the outlaw biker of the marine world: intelligent and opportunistic, a stone-cold cannibal willing to attack divers with a seemingly deliberate hostility.

What about the giant squid, you may ask? "Wimpy," says Cassell. The giant—which grows to 60-plus feet and is one of only four squid, out of the 400 or so species found in the oceans, that are human-size or bigger—is generally considered to be fairly placid. In any case, it's so elusive, no modern squid hunter has ever even seen one alive. No, if you want a scary squid, you want a Humboldt. And they're easy to find, teeming by the millions in Pacific waters from Chile to British Columbia. (It's named after the Humboldt Current, off South America's west coast.)

Cassell first heard about the "diablos rojos," or red devils, in 1995, from some Mexican fishermen as he was filming gray whales for German public television in Baja's Laguna San Ignacio. Intrigued, he made his way to La Paz, near the southern tip of Baja, to dive under the squid-fishing fleet. It was baptism by tentacle. Humboldts—mostly five-footers—swarmed around him. As Cassell tells it, one attacked his camera, which smashed into his face, while another wrapped itself around his head and yanked hard on his right arm, dislocating his shoulder. A third bit into his chest, and as he tried to protect himself he was gang-dragged so quickly from 30 to 70 feet that he didn't have time to equalize properly, and his right eardrum ruptured. "I was in the water five minutes and I already had my first injury," Cassell recalls, shaking his head. "It was like being in a barroom brawl." Somehow he managed to push the squid-pile off and make his way to the surface, battered and exhilarated. "I was in love with the animal," he says.

After his initial squid beatdown, Cassell built himself some homemade fiberglass body armor modeled after the storm­trooper armor in Star Wars. "I realized I could survive the worst attacks with armor alone," he says. He's been observing and filming the Humboldt ever since, over the course of more than 300 dives, and was featured in the Discovery Channel's 2005 Killer Squid program. As far as he's concerned, for raw power, aggression, and evolutionary perfection, nothing else can compete. "They are one of the most beautiful creatures, and they just happen to be lethal," Cassell says. "There is no life form on this planet more alien than a Humboldt squid."

Cassell is in Santa Rosalía this time around to shoot additional high-definition footage for a documentary he's putting together called Demonio Rojo: The Truth About the Humboldt Squid. His girlfriend, Shawna Meyer, 32, is with us to keep everything organized, and Dale Pearson, a 37-year-old part-time contractor who's been running dive and spearfishing trips to Baja on the side for four years, is flying in to help with the filming and to make his first Humboldt dive. Cassell and Pearson have formed a partnership called Sea Wolves Unlimited and plan to start squid-diving expeditions. I'm their first guinea pig.

The next day, motoring out in two 25-foot home-built pangas with Guerito Romero, a 27-year-old local fisherman, I ask him what he thinks of our dive plans. "Scott es muy crazy," he says, laughing. Then he starts pinching me, mimicking all the squid bites he thinks I'm in for.