The sky is an immense bowl of blue and the boiling-hot sun speckles the flat green waters gold as our boat edges out of Port Douglas, Australia, just north of the tropical resort town of Cairns. On board, tourists from around the world are heading for the Great Barrier Reef, the world’s largest natural structure—stretching for 1,400 miles along the continent’s northeastern coastline.

After a 90-minute journey, we arrive at Opal Reef, a chunk of the Great Barrier Reef five miles long and three miles wide, where frothy white waves break gently over shadowy outcrops of coral as big as houses. We throw on our snorkeling gear and slip quickly into the water, drifting over the shallow reef facedown, entranced by the multihued coral—some resembling giant brains, others massed like stag horns and mammoth fans—and the neon-colored small fish that dart in and out. Not one of the crew members has bothered to warn us that Opal Reef is where one of the planet’s deadliest creatures killed a visitor three years earlier.

Robert King, 44, from Columbus, Ohio, was snorkeling over the same underwater landscape when he felt a mild sting on his chest and came back onto the boat. Within 25 minutes his face flushed tomato-red as severe pain gripped his stomach, chest and back muscles. The skipper radioed for a medevac chopper, whose crew injected King with a massive dose of pethidine, an opiate-like painkiller, then winched him from the boat and rushed him to Cairns.

By the time he was wheeled into the emergency ward at CairnsBaseHospital, King’s speech was slurred. He was put on a ventilator, as doctors pumped him full of painkillers while racing to save his life. A local zoologist, Jamie Seymour, was called in to take a scraping of the sting site. While he worked, Seymour noticed that King’s blood pressure was spiking dramatically. King lost consciousness; then, Seymour says, “an artery or vein in his brain blew.” Blood flooded King’s brain tissues, and two days later he died.

After analyzing the shape and size of the stinging cells, which were about an inch long, Seymour blamed King’s death on a nearly transparent jellyfish the size of a thumbnail. Covered from the top of its head to the tip of its four tentacles with millions of microscopic spring-loaded harpoons filled with venom, it’s one of at least ten related species of small jellyfish whose sting can plunge victims into what doctors call the Irukandji syndrome. “The symptoms overwhelm you,” says Seymour, 40, who himself was stung by an irukandji on the lip, the only part of his body uncovered as he scuba-dived looking for specimens near an island off Cairns in late 2003. “On a pain scale of 1 to 10, it rated between 15 and 20,” he says, describing the vomiting, the cramps and the feeling of panic. “I was convinced I was going to die.” But he was lucky; not all species of irukandji administer fatal stings, and he recovered within a day.

So far, only King’s death—and perhaps that three months earlier of an Englishman, 58-year-old Richard Jordan, farther south on the Great Barrier Reef—can be attributed to irukandji venom, but Seymour cites research suggesting that because the symptoms may resemble strokes or decompression sickness, and can lead to drowning, countless more swimmers have probably fallen victim to Irukandji syndrome in offshore waters throughout the tropics. Stings from the irukandji species who live in waters closer to shore are rarely fatal but are still excruciatingly painful: for centuries before the tiny jellies were identified as the culprit, the local Aborigines at Cairns, the Irukandji tribe, knew that to swim in the shallows in the rainy season, from November to May, was to risk getting stung, although they didn’t know by what.

More ominously for residents of North America, doctors at the U.S. Army Special Forces Underwater Operations School at Key West, Florida, have treated military divers suffering from symptoms similar to the syndrome; U.S. Navy divers have seen Irukandji-like jellyfish in the waters off Cuba’s GuantánamoBay; swimmers have been badly stung in Hawaii; and the Gulf of Mexico and the adjoining southern U.S. Atlantic coastline have seen an increase in people being sickened by stings that almost certainly come from an irukandji or a related jellyfish.

Most jellyfish are passive; they drift up and down in the water column, or are pulled to and fro by the tides and winds. They float through the oceans devouring tiny fish and microscopic creatures that bumble into their tentacles, and are no threat to humans.

But those known as box jellyfish, for the shape of their bell, or body, are a breed apart. Also called cubozoans, they’re voracious hunters, able to chase prey by moving forward—as well as up and down—at speeds of up to two knots. They range in size from the various irukandji species to their big brother, the brutish Chironex fleckeri, which has a bell the size of a man’s head and up to 180 yards of tentacles, each lined with billions of cells bursting with deadly venom. Also known as a sea wasp or marine stinger, Chironex, which is far deadlier than irukandji, boasts powerful stingers, or nematocysts, strong enough to pierce the carapace of a crab and quick enough to shoot out at the fastest speed known in the natural world—up to 40,000 times the force of gravity. And unlike other jellyfish, a box jellyfish can see where it’s going and alter its course accordingly; like an eerie creature sprung from science fiction or a horror movie, it has four separate brains and 24 eyes, providing it a 360-degree view of its watery world.

“A Chironex fleckeri can kill a human in one minute flat,” says Seymour, widely considered the world’s foremost box jellyfish researcher. The most recent victim was a 7-year-old boy who died two years ago at a beach south of Cairns, becoming one of about a hundred people believed to have been killed over the past century by Chironex in Australia alone. (No one knows for sure how many swimmers have died from the stings of other box jellyfish outside Australia, but Seymour puts the number at “hundreds, possibly thousands.”) Survivors, those lucky enough to have been gripped by less than the four yards of Chironex tentacle that can kill an adult (or the two yards it can take to kill a child), suffer pain that one has described as “like having a bucket of fire poured on me” and are branded by macabre tentacle marks, scarlet tangled wheals that make the victims look as if they’d just been lashed at the mast. “Chironex is by far the world’s most venomous creature,” says Seymour. “It makes venomous snakes look like amateurs.”

And it wreaks havoc with the November-to-May swimming season throughout northern Australia, where fear of it closes almost all of the beaches along the entire top half of the continent from Gladstone in the east to Exmouth in the west. At the few beaches that remain open, swimming areas are enclosed by netting that keeps out the deadly jellies, and lifeguards wear neck-to-ankle Lycra suits. Signs warn swimmers not to rub a sting, but instead to douse it in vinegar, which immediately kills any stinging cells not yet activated.

When the deaths of Robert King and Richard Jordan threatened to further dampen the multi-billion-dollar Great Barrier Reef tourism business, the Queensland state government swiftly set up the Irukandji Jellyfish Response Taskforce, made up of leading marine biologists, zoologists, toxin specialists, emergency-ward doctors and lifeguards, to begin to find out as much as possible about the tiny jellyfish. From her lab at JamesCookUniversity in Townsville, task force member Lisa-ann Gershwin, a 41-year-old California stockbroker-turned-jellyfish taxonomist, drives four hours north to Cairns each December to catch irukandji.

“We hardly know anything about their lifestyle, how they breed, where they come from, how fast they grow, how long they live, or even how many species there are,” she says when I join her and a team of marine biologists at Palm Cove, an idyllic curve of tropical sea nudging pristine sand near Cairns and the site of more irukandji stings than any other beach along the northeastern coast. “But they’re like other cubozoans: they’re really neat, like aliens. They split from the other jellyfish, the scyphozoa, more than 300 million years ago, long before dinosaurs walked the earth, and have been making their own way along the evolutionary path ever since.”

Gershwin and her team have gathered at Palm Cove for the annual irukandji bloom, when a huge number of the jellyfish swim or drift into waist-deep water by the beach to feed. On the day after Christmas, we pull on neoprene wet suits that cover us from toe to neck, put on rubber diver’s booties and gloves, seal the wet suits around our wrists and ankles with duct tape, and wade into the water. There, we trudge back and forth in the shallows under the boiling summer sun, nets hooked to our shoulders like plow horses, to collect seawater in cylinders about the size of large soda bottles.

Hour after hour of sweaty torture produces only plankton, tiny larval fish and salps—invertebrates about half-aninch long that tend to turn up in the shallows just before the irukandji bloom. Finally, at mid-evening, Gershwin pours the water from yet another cylinder into a transparent bowl. A few moments later she screams, “We’ve got one!” We rush to join her on the beach as she shines a flashlight on the bowl, revealing a jellybean-size box jelly known as Carukia barnesi, dangerous but usually not fatal. Head down, it swims purposefully around the bowl as if seeking escape, its ability to move unlike any other jellyfish I’ve ever seen before.

No one even knew what irukandji looked like in the 1950s, when a Cairns doctor, Jack Barnes, went searching for whatever it was that stung, and then sickened, hundreds of people at Queensland beaches each summer. Over several years, he tested on his own body the sting of every jellyfish he could collect from beaches in and around Cairns, but none produced the Irukandji syndrome. Then, one day in 1961, he found a tiny jellyfish of a kind he’d never seen before.

As a curious crowd gathered around him, he asked for volunteers to be stung. The first to step forward was his own 9-year-old son, Nick. “I said, ‘Try it on me, Dad, try it on me,’ ” Nick recalled years later in an interview with the Sydney MorningHerald Magazine. “So, he ended up stinging me first, then himself, then a big local lifeguard called Chilla Ross.”

The three returned to the Barnes family home where, 20 minutes after being stung on the beach, they began to feel the venom’s terrifying effects. Chilla Ross began screaming, “Let me die.” Nick remembers vomiting “as Dad carried me upstairs, then I was lying on a bed swallowing painkillers. I felt pretty terrible”—so terrible, in fact, that he found himself “thinking that dying mightn’t be a bad idea.” But he survived, as did Ross and his father. Three years later, Jack Barnes described the ordeal in the Australian Medical Journal, writing that all three of them had been “seized with a remarkable restlessness and were in constant movement, stamping about aimlessly, swinging their arms, flexing and extending their bodies, and generally twisting and writhing.” In honor of Jack Barnes’ discovery, the creature that stung them was given the scientific name Carukia barnesi.

Ken Winkel, director of the Australian Venom Research Unit, has conducted experiments on anesthetized and ventilated piglets and concludes that Carukia barnesi venom “fires the sympathetic nerves, pushing up dramatically the blood pressure and heart rate. That’s why you get sweating, nausea, anxiety and a feeling of doom”—the latter effect caused, Winkel believes, by the triggering of the stress hormones adrenaline and noradrenaline. In the body, noradrenaline produces a heart-thumping, throat-tightening, fight-or-flight effect. It’s what you would feel, Winkel says, “if you were put in a cage with a hungry lion.”

Chironex venom, by contrast, attacks the heart directly, which can cause dramatic and rapid cardio-respiratory arrest, says Darwin-based professor Bart Currie, a specialist in treating Chironex victims. “Ahealthy heart contains millions of muscle cells that all beat to the same rhythm to pump blood through the body,” he says. “For reasons we don’t yet know, Chironex venom makes the heart cells beat irregularly. If enough venom is injected, the heart shuts down altogether.”

Death comes quickly to Chironex victims because—unlike venomous snakes, which inject a glob of venom that must pass through the lymphatic system before draining into the rest of the body—Chironex shoots its venom into the bloodstream, giving the venom a direct pathway to the heart.

In addition to their stinging cells, box jellyfish have another superlative weapon in their hunt for prey: one of the world’s most effective sets of eyes. On a windy day at a beach 40 miles north of Cairns, I help a team led by Dan Nilsson, a zoology professor at Sweden’s Lund University and a renowned expert on animal eyes, in catching ten specimens of a box jelly about the size of a coffee cup. While the species, as yet unnamed, is less deadly than Chironex or the offshore species of irukandji, in 1990 its close relative Chiropsalmus quadrumanus stung to death a 4-year-old boy in the shallows of a beach near Galveston, Texas. Chiropsalmus quadrumanus has also been reported in the waters off North Carolina, Brazil, Venezuela and French Guiana.

Like the irukandji at Palm Cove, the jellyfish we capture skeeter around the bucket of seawater that Nilsson puts them in, carefully skirting its curved sides. “They swim like fish, not like jellyfish,” he says with a smile. He plucks one from the bucket and shows me what keeps it from bumping into things: four tiny black dots, containing the jellyfish’s 24 eyes, on strands connected to each side of the cube of jelly. Under the microscope, Nilsson has detected in each dot something he calls a sensory club, which is an organ with a set of six eyes, including four that are—much like the eyes of other jellyfish—simply pits, limited to detecting light intensity in various directions. But the two other eyes in each sensory club have more in common with human eyes than the eyes of other jellyfish, with lenses, corneas and retinas. One eye, which points obliquely downward at all times, even has a mobile pupil that opens and closes. The other major eye points upward. “We’re not exactly sure what these eyes are doing,” Nilsson says, although he believes they may help the jellyfish “position itself in the right place where there is plenty of food.” They also help the animal situate the shoreline and the horizon—to avoid being dumped on the beach by a wave—and see obstacles that would tear its delicate tissue, such as a coral reef, a mangrove tree or a pier.

Nilsson has collected and studied the eyes of box jellyfish in other locations, such as mangrove swamps in Puerto Rico, and has found exactly the same set of 24 eyes in box jellyfish wherever he’s gone. “They live in very different habitats,” he says—“some in mangrove swamps, others in sandy beaches, some on rocky shores, coral reefs and kelp forests. Exactly why they have the same eyes, we don’t know.”

They also have the same stomach—or, rather, stomachs. Because a box jelly, as Jamie Seymour puts it, “charges around the ocean all day hunting mobile prey, prawns and fish,” its metabolic rate is ten times that of a drifting jellyfish. So, to swiftly access the energy it needs, the box jellyfish has developed a unique digestive system, with separate stomachs in each of its tentacles. All box jellies turn their food into a semi-digested broth in the bell, and then feed it down through the tentacles to be absorbed. Since a Chironex can have up to 60 tentacles, each as long as 3 yards, in effect it has up to 180 yards of stomach.

If box jellyfish eyes are a puzzle, its four primitive brains—positioned on each side of its body and attached to it by the same strand that anchors its eyes—are an enigma. Can the four separate brains communicate with each other? If so, do they merge the images they receive from the 24 eyes into one image? And how do they manage if different eyes detect radically different images? Nilsson shrugs. “They’ve evolved a rather advanced system unlike any other animal on earth,” he says. “But we have no idea what’s going on in their four brains, and I suspect it will be a long time before we find out.”

Six months ago, after tagging Chironex in the wild with tiny ultrasonic transmitters that allow him to track an individual jellyfish for up to three weeks, Jamie Seymour made an announcement that startled his fellow scientists. “During daylight hours, from about six in the morning to three in the afternoon,” he said, “they moved in straight-line distances of about 250 yards an hour. But from 3 p.m. to 6 a.m. the next morning, they moved an average of less than ten yards an hour.”

Determined to see the phenomenon for himself, Seymour donned a wet suit and plunged into shallow water off a beach south of Cairns. There, he observed Chironex resting motionless on the seafloor, their bells not pulsating and their tentacles completely relaxed. When he shined lights on them, they rose, swam around for a short time, and then settled back on the seafloor. Sleeping!

“It makes a lot of sense for them to become inactive at night when they can’t see their prey,” says Seymour. “They decrease the energy used in locomotion and divert it to growth.” But not all researchers accept that Chironex do, in fact, sleep. And because the box jellyfish brain is so radically and impossibly different from the makeup of all other brains on our planet, we may never know who is right.

While scientists struggle to untangle the biological secrets of box jellyfish, doctors are having increasing success in treating the damage they do to humans. An antivenin for Chironex stings—made from antibodies created in sheep that are injected with the venom—is now administered to victims in northern Australian hospitals. There is no antivenin yet for the Irukandji syndrome, but Lisa-ann Gershwin is edging toward an important breakthrough—the first-ever mass breeding of tiny box jellyfish in a lab, from specimens she caught at Palm Cove this year. So far she’s managed to breed just a handful of the “up to a million” jellyfish that she says researchers like Ken Winkel need to develop an effective antivenin.

More promising for serious irukandji stings, at least in the short term, is a treatment being used in the TownsvilleHospital’s intensive care unit: the infusion of a solution of magnesium sulfate directly into a victim’s veins. “We’ve seen it swiftly reduce, to safe levels, the hypertension, and it lessens the pain considerably,” says Michael Corkeron, one of the unit’s physicians. But, he cautions, “we still have more to learn, including the correct dosage, before magnesium becomes standard treatment.”

So until a fail-safe cure is found, box jellyfish, from the tiny irukandji that killed Robert King to the huge Chironex, will continue to cause illness and death in tropical waters worldwide. Says Jamie Seymour: “All we can do is alert people to the danger, here in Australia and overseas, and make sure that anyone stung is treated as quickly as possible. Then it’s in the lap of the gods.”