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.

“I'm really scared of only one or two things,” he declared with mock grandeur. “Global thermonuclear war would be one. Humboldt squid would not.”

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.

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 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.

If you're chasing after monster Humboldts, Santa Rosalía is an oceanic game park. For most of its 120 years, it was a copper-mining town, and you can still see the rusting machinery and gaping streetside mine shafts of that Dickensian time. But the mine shut down for good more than 20 years ago, and today Santa Rosalíans mine another natural resource, Humboldts, which are particularly abundant in the summer. Every night the squid come around, a fleet of more than a hundred pangas races out onto the Sea of Cortez, and Santa Rosalía's hardscrabble pangeros start handlining for Humboldts at depths of up to 1,000 feet. By morning, if luck is with them, each panga is offloading up to a ton of squid at the local processing plant. The catch is packed up and shipped to Asia, where it's a staple in local cuisine. (It's a sustainable fishery as long as the pangeros stick to handlining, says Gilly, partly because hammerhead sharks and big sport fish in the Sea of Cortez—which prey on squid—have been fished down, leaving the Humboldt to thrive.)

Down at the waterfront, Cassell, with his dive gear and cameras, is a glaring oddity, but by now the pangeros are familiar with the eccentric gringo who likes to commune with the "calamar gigante." As the Baja sun drops toward the hills, Rafael Garibaldi, whose pangas Guerito has lined up for us, mans the outboard and motors us out past the breakwater. Cassell is quiet, checking over his gear, including the restraining cables we'll use to prevent the squid from dragging anyone deep. Shawna, who's been with Cassell since 2004 and works for a plumbing-supply wholesaler when she isn't on one of his "missions," is perched in the bow fiddling with the video camera she'll use to film any topside action. And Dale, a crack spearfisherman, is jumping around in hyperactive anticipation of his first Humboldt dive. Dale is the quintessential California water rat: blond, brash, and hilarious. He's got a big whitetip shark tattooed across the back of his neck and the obligatory shark tooth hanging from a silver chain. "I'm really scared of only one or two things," he declares with mock grandeur. "Global thermonuclear war would be one. Humboldt squid would not."

The squid are all around me. Everywhere, ghostly shadows torpedo through the water.

We motor for 20 minutes before Rafael brings the panga to a halt. Cassell clamps two anti-mugging cables to the gunwale and throws them over the side. They drop underneath the boat, dangling to 50 feet. Another cable, this one 70 feet long, is attached to the bow, and Rafael and Guerito pull on gloves and go to work. Rafael, 50, has a deeply lined face and a drooping mustache. He throws a squid jig over the side. It's an utterly wicked-looking footlong device, fashioned from a glow-in-the-dark plug (squid are visual predators) that bristles with multiple tiers of closely spaced needles. This afternoon's mission: bring squid up on the jig for Cassell to release, so he and Dale can film them—and any trailing cannibal Humboldts—up close.

It's a matter of minutes before Rafael utters a soft grunt to announce that he's got one. He starts swinging his arms rhythmically, working to recover more than 500 feet of line. Cassell pulls his mask down over his face, rolls backwards off the panga, and disappears into the darkening water. Dale follows. They clip in; neither is wearing squid armor, trusting that the presence of more than one diver will keep the squid cautious. Rafael keeps hauling, dragging the monofilament across the gunwale, where years of fishing has burned deep grooves into the wood. The squid finally hits the surface, and even though, at about four feet, it's not a big one by Humboldt standards, I'm surprised by its heft and the defiant fire-hose spurt of water and ink that gushes into the air as Rafael pulls it clear of the surface. The squid is a deep, angry red, its arms are whipping, and its black, silver-dollar-size eyes are eerily human as they seem to appraise us. Rafael drops it back into the water, and I can see the camera lights circling underneath as Cassell and Dale film.

Rafael almost immediately hooks another Humboldt. With a slight smirk, he waves me over and offers me the line. I start hauling. After just 30 seconds, the muscles in my lower back are on fire. I try not to slow my pace, but I can't fool Guerito and Rafael, who are getting a big kick out of watching the gringo suffer for a single squid. Suddenly the line starts jerking hard, like when a bluefin tuna starts to run. "It's trying to get away," I tell them in fractured Spanish. "No," Guerito says, with an evil grin. "The others are eating it."

The next day, it's my turn to get wet. I remind myself that Cassell is still alive after hundreds of dives, but it's hard to ignore two important qualifiers. First, he's a much more skilled diver. And second, he's a hell of a lot tougher.

I try to channel Gilly, but instead I feel like I have kerstitch emblazoned across the back of my wetsuit as we motor out early in the afternoon. Today, there's a building breeze, and the sea surface is choppy, which will reduce visibility. Still, we gear up (I also pass on the unfamiliar squid armor after getting assurance from Cassell that he'll watch my back) and drop into the water as soon as Rafael finds the squid shoal. No matter what happens, I tell myself, do not let your regulator get yanked from your mouth.

Cassell escorts me down to about 45 feet and clips one of the restraining wires to my scuba backpack. I settle in and take a look around. The rays of sun shafting into the depths give the water a greenish, hazy hue, so I feel as if I'm swimming in a light fog. There's a current running, and we're quickly strung out on the wires, making it difficult to maintain depth as Rafael starts hauling jigged squid from below us.

I see a jig rising past me, and the hooked squid is flashing red and white like a neon sign. It feels like fear—or at least a vain plea for mercy.

With the current, the poor visibility, the confusion of wires, and a barbed jig whizzing up from the depths (Cassell was once hooked in the chest, and even he admits that it really hurt), I forget for a moment to worry about gangs of squid. Instead, I concentrate on staying clear of Cassell and Dale as they set up beneath the boat and making sure I don't get hooked by Rafael. Slowly I notice that the squid are all around me. None of them decide to take the rookie for a ride or even move in for a tentacular taste test. But everywhere on the periphery of my vision, ghostly shadows are torpedoing through the water. Their speed and agility is mind-blowing, and every once in a while one will suddenly eject a cloud of rust-colored ink into the water. Cassell later explains that the squid mix mucus with the ink, which allows them to give the burst a squidlike shape to fool predators. He then shows me a video sequence filmed that day in which a jigged squid inks just as a cannibal is rushing in and dupes the attacker into hitting the cloud.

I see a jig rising past me, and the hooked squid is flashing red and white like a neon sign. It's a stunning display, and another extraordinary aspect of Humboldt squid behavior. Both Cassell and Gilly believe that the squid use the flashing (which is enabled by millions of chromatophores in the skin that can be opened like little umbrellas to show red or closed to show white) to communicate with one another, though they've deciphered none of the "language." The effect is so dramatic you can almost feel the squid's emotion being transmitted through the water. In this case, it appears to be fear—or at least a vain plea for mercy. Because as the jigged squid goes past, I can see a pack of four-foot cannibals shadowing along, jetting in mercilessly and relentlessly to take quick bites of flesh.

When I eventually roll back into the boat untouched, the brief experience has already started to transform the squid in my mind from a fearsome unknown into an astonishing example of evolutionary design. On the way back into Santa Rosalía, I start to wonder whether all the talk about red devils and killer squid is more creative marketing than reality. When I spoke with underwater cinematographer Bob Cranston, he admitted that nature television likes to exaggerate the dangers of the Humboldt squid. "There's a lot of hype," he said. "But I empathize with Cassell. We've all got to make a living."

I voice my doubts to Cassell. "Well, we've just been in with the babies," he says. "You really need to see the big guys." I ask whether he has film of his deep encounters with the six- and seven-foot "rogues" and "giants" he likes to talk about and whether he can send me clips of his hairiest confrontations. I like the idea of monsters abroad in our risk-managed, hyper-conditioned world, but now I want to see proof.

A few days after I return home, a DVD arrives from Cassell. It reads humboldt squid clips, and the letters are burned into a picture of Cassell sitting in a panga with a Humboldt across his lap. It's not just any squid. It's thick and muscular, perhaps twice the size of anything we saw.

"OK," I say to myself, and cue up the DVD. I scroll my way past clips of Cassell pimping in his Star Wars squid armor for the Discovery Channel and telling spooky tales to get people squirming in their Barcaloungers. I check out a sequence called "Attacks," in which flashing squid take runs at Cassell, their arms grating across his camera gear as he grunts and pants for breath. Finally, I find a segment called "Giants" and settle in.

Cassell is in a pale-green underwater world, a couple hundred feet down, and directly in front of him is an utterly massive squid. (Cassell estimates that it was about eight feet long and perhaps 200 pounds.) Its skin is pocked by scars from numerous battles, and its body appears unusually heavy. Dozens of other large squid are swimming nearby, flashing their eerie messages, and none of them seems the least bit intimidated by Cassell. Indeed, they repeatedly jet in to grab at him. Sometimes the probes come head on, the squid lining up like darts and then bull-rushing him with a flare of arms and tentacles followed by the scrape of sucker teeth on his armor. Sometimes there are sneak attacks from behind, signaled by grunts and yelps from Cassell as he tries to free himself. During one attack, you can hear Cassell laughing maniacally before finally conceding an "Owww."

He's in a world that would be terrifying to the average diver. But he's loving it. The mega-squid, which Cassell will later dub Scar, appears again and again to eyeball Cassell up close and probe him with its tentacles. Scar seems almost imperious in his disdain for this human invader, and Cassell will later write, "It occurs to me that this might well be the first encounter of its kind for both species. I can only describe it as a dance. A dance of peace, curiosity, and discovery."

Who knows how well Cassell really understands these animals and whether a human can ever really dance with a squid. But Cassell is a romantic. What's important to him is that in those moments beneath the Sea of Cortez, he transcends the human world and dives his way into an undersea realm that is wild, often brutal, and ruled by a spooky alien species. The monsters are really there; you just have to dive deep enough to find them. And that's something most of us will never do.

Correspondent Tim Zimmermann's (@Earth_ist) August 2005 story on cave diver Dave Shaw was a National Magazine Award finalist.