“Smell this!” Mary said, pressing the flower to Gil's nose. “I wish we'd had some of these at the wedding.”

He sniffed cursorily and turned his face away. "Let's go find a beach."

"Why don't we stay here awhile first?" Mary said. "We can find beaches anytime. But this"—she waved the flower up toward the leafy, cathedral-like rainforest canopy, the thick vines dangling from the arching branches, the orchids sprouting from tree trunks, the tangled profusion of life—"this place is extraordinary! It looks like The Land Before Time."

"I'm heading back to the car," Gil said.

"How can you expect to appreciate the beauty of a place if you refuse to spend some time in it?"

"I don't consider this place beautiful," Gil replied. "I think it's malevolent. And besides, there are ants. Let's go."

Mary dropped the flower. "You always want to be somewhere else, Gil," she said. "Why can't you just enjoy where you are?"

But he'd already started to walk back along the overgrown path, angrily shaking his sandaled feet to dislodge the crawling ants.

The honeymoon had been rough. They seemed to fight at least once every day, each blowup followed by a long, dripping silence. It hadn't been like this back in the States. They'd met at a birdwatching camp on the New England coast when her first marriage had just ended and his was about to end, the victim of too much time devoted to his law practice. By the time the trip was over, they'd arranged their first "date," a four-day snorkeling expedition to the Bahamas. He proposed to her at sunset on the fourth day, and they married just three months later. It seemed only appropriate that for their honeymoon they should choose one of the world's most exotic places: the Cape York Peninsula, a lush spit of land that projects like a sharp spine from the northeast coast of Australia into the tropical waters of the Coral Sea.

They drove in silence along the coastal highway. To the right, through breaks in the low forest that fringed the beach, they caught glimpses of the sea—calm and blue-green. "When do you want to go out to the reef?" Gil asked finally. It was his way of offering to make up.

"How about tomorrow?" Mary replied. "We could go to the beach today and tonight ask the hotel desk clerk to arrange a boat."

It was her way of accepting.

Gil slowed the car where a sandy track led from the road toward the water. "How about here?"

"Oh, Gil, it's perfect!" The curving strip of yellow sand glistened and the water radiated a vivid aquamarine. They climbed out of the car and walked down to the beach. They couldn't see another sign of human life along the three-mile crescent. Gil wrapped his arms around Mary and hugged her.

"I'm so happy to be here with you," she whispered into the soft blue fabric of his T-shirt. She was quite sure she meant it.

"Me, too," he said into her sweet-smelling, honey-blond hair. They'd get used to each other eventually.

They dropped their packs, pulled out their towels, and spread them on the warm sand.

"The water looks lovely," she said. "Let's go for a swim."

"I don't know about swimming here," Gil replied, sitting down abruptly on his towel. "There aren't any enclosures."

Back at the popular beaches near their hotel they'd seen people swimming inside what the locals called "stinger nets." These were floating, corral-like enclosures made of fine netting designed to keep out jellyfish. As they strolled along the beach, they'd read the sign posted at one enclosure: WARNING: MARINE STINGERS ARE DANGEROUS OCTOBER TO MAY.

"I saw people swimming outside the enclosures," Mary said. "They say the worst of the jellyfish season is over."

"I still don't think it's a good idea," Gil said.

"Gil, how can you let this incredible water go to waste? Here we are in paradise and you're sounding like a lawyer."

"I'm just trying to be prudent in a place where we don't know the score."

She stripped off her shorts and T-shirt and let them drop to the sand. Underneath she wore an aqua bikini. She started walking toward the water, propelled as much by her defiance of his oppressive caution—of his whole being—as by the tempting blue sea.

"You're being foolish, Mary."

When it comes to predatory animals, humans have little to fear but themselves. We kill one another at a rate of more than one million per year, mostly wartime casualties. The second-deadliest threat is snakes, which kill over 100,000 people annually, followed by crocodiles (960), and tigers (740). The much-feared shark falls far down the list—only about seven human victims annually worldwide—making it a lightweight compared to the ostrich, which when cornered can kick viciously with hammerlike feet and sharp talons and kills some 14 people every year. As for the ferocious grizzly bear, it ranks about the same as mustelids (weasels, badgers, skunks, and the like), which kill an average of four humans a year, primarily pet ferrets attacking unattended babies. A couple more reassuring statistics: In the United States and Canada, you have more to fear from moose (six deaths per year) than any other creature except snakes (12). And the most likely place in the States to be attacked by an alligator (three deaths between 1992 and 1998) is not deep in some swamp, but on a golf course.

Very few animals stalk humans as prey, and those that do, such as the infamous man-eating tigers of India and lions of Africa, tend to be individual animals—typically outcast young males searching for new territory—that lose fear of humans and develop a taste for their flesh. Animals, for the most part, attack humans only when they are surprised or feel threatened or when defending their offspring. Snakes kill far more humans worldwide than any other animal, but as one authority states, they "have never been shown to attack without provocation, despite lengthy historical commentary to the contrary." Most bites occur in tropical countries when a rural villager unwittingly disturbs a snake, often by stepping on it in the dark. In the United States, by contrast, many victims are under the influence of alcohol or drugs and, in the words of another expert, are "messing with" the snakes. Researchers in Alabama have noted a statistical drop in venomous snakebites among adult men when University of Alabama or Auburn University football games are televised, presumably because the men are ensconced in their TV rooms.

There are no statistics showing that one region of the world is more dangerous than another in terms of animal attacks. Still, one can speculate. It would seem that parts of Africa inhabited by big game would make the list, as would the snake-infested Amazon Basin and parts of Southeast Asia such as Vietnam's Mekong Delta, where in one area studied by researchers, cobras, kraits, and vipers kill more than 2,700 people each year.

On the list, too, one expects, would be the coast of Australia and the region around Cape York Peninsula, which is home to an assortment of sharks, venomous snakes, poisonous fish, and the deadly saltwater crocodile. Of all those faunal hazards, one creature, despite its diminutive size, towers above the rest. This is a graceful-looking jellyfish not much larger than a grapefruit, known by the scientific name Chironex fleckeri, generically called a box jellyfish. As writer Bill Bryson noted in his recent best-selling book about Australia, In a Sunburned Country, Chironex is perhaps the most lethal venomous creature on earth. At least 63 Chironex deaths have been recorded along the coast of northern Australia since 1900, though many more deaths have probably gone undocumented through other parts of the South Seas. By some estimates, the venom of a Chironex can kill a human in less than a minute.

Mary waded deeper. The rippled sand massaged the bottoms of her feet and the warm tropical water soothed her skin like a mineral bath. The sea extended like a huge, placid lagoon toward the Great Barrier Reef, some 30 miles offshore. She badly wanted to see the reef, its spectacular corals and fish. It was hard enough to convince Gil simply to go for a swim; how difficult would it be, despite their plans, to actually drag him out there? She kept wading. She knew she wasn't being prudent, but the water looked fine, and his annoying, persistent caution made her want to go deeper, away from him. Was this marriage a mistake? she wondered. Already the fights she had with Gil resembled the same shopworn arguments she'd had with Tom, her ex-husband. If Gil turned out to be like Tom, cautious and unimaginative, she'd leave him. Maybe first have the child she wanted, and then go.

"Come on in," Mary shouted, up to her hips in the clear water. "See, there's nothing to be scared of."

Gil waved her off. "I'm perfectly happy sitting here," he said.

But he wasn't happy. How could he be happy with someone who was pushing him all the time? A constant, tiny shove-shove-shove. This was supposed to be a vacation, wasn't it? He pushed himself hard enough at work; he didn't need someone needling and telling him to loosen up. Mary now reminded him of his ex-wife, Betsy, who for five years had chided him to work less, travel more, go for hikes and picnics and visits to galleries. She was relentless. He'd finally moved out. Then, strangely, he found himself taking up some of the activities he'd resisted for so long, as if to prove to her that he wasn't the stodgy person she thought. It had been a great relief to meet Mary at the bird-watching camp—she was so patient and understanding when he told her about his failed marriage—but now it seemed he was reliving those five bad years. Couldn't she just relax a bit instead of charging from rainforest to reef to outback? He didn't want to think about how it would be when they got back home. He'd already made the mistake. How long would a divorce take? And, more important, what would it cost him?

"Last chance!" she called, twisting her head toward him.

"Hurry up and swim if you're going to swim," he answered, his irritation rising. "Otherwise let's go back to the hotel."

She brought her arms over her head and sprang gently off the sand with her toes. As she dived underwater, sliding in with a gentle splash, she made her decision: That's it. We're finished.

About a year earlier, late in the October­–April wet season, two spawning Chironex fleckeri released sperm and eggs into a river estuary not far from Gil and Mary's isolated beach. Joining in the warm water, the two cells soon grew into a minute ball—the planula, as this stage is called—and dropped to the river bottom, attaching to the underside of a rock. There the planula sprouted the beginnings of a crown of tentacles and grew into a tiny polyp that by the end of the dry season had metamorphosed into a small jellyfish. Just ahead of the monsoon rains, the Chironex propelled itself out of the estuary and into the Coral Sea.

For the next few months it gently pulsed through calm waters along the coast, feeding and growing, avoiding violent currents and waves that could tear its delicate tissues. Its body, an almost-transparent milky white, 95 percent water, developed into a graceful bell shape with a squarish, four-cornered bottom rim—thus the name box jellyfish. By filling its bell with water and squeezing it out like an umbrella opening and closing, the jelly could jet along at speeds up to four miles per hour. A limb shaped like a chicken's foot dangled from each of the four corners of the bell's rim and from these grew the jelly's tentacles, as many as 15 per limb. Only a quarter of an inch in diameter and stretching more than 15 feet when fully extended, the tentacles resembled twisted lengths of skinny—and highly charged—electrical conduit.

Using primitive eyes to help it spot large objects, the jelly followed schools of shrimp that congregated just off the sandy beaches. Sensitive to strong sunlight, it lingered near the sea floor during the height of the day, rising toward the surface as the sunlight weakened in late afternoon, trailing its long tentacles behind it, trolling for prey. When a shrimp or a small fish came past, inadvertently brushing a tentacle, it died almost instantly. The Chironex would then reel in the catch, feeding the meaty morsel into wide, grasping lips.

Mary never saw it.

Plunging beneath the smooth surface, she glided underwater, savoring the trickling sound in her ears and the warmth of the tropical sea, relieved that she'd made the decision to leave Gil. Then something brushed against her arms. She flinched, startled. Was it seaweed? Then it brushed against her shoulders, her midriff, her back.

Reflexively, the jelly's tentacles contracted, piling themselves in loops onto her skin in an instinctive attempt to apply maximum surface area—and thus maximum venom—to the victim. Each tentacle was studded with millions of tiny venom sacs, called nematocysts, the entire jellyfish armed with an estimated five billion of them. The touch of Mary's skin—indistinguishable, as far as the jelly was concerned, from a shrimp or a fish—triggered the release of coiled tubules with sharp tips that sprang from the top of each nematocyst like a jack-in-the-box. The tubules, each about .03 inches long, jabbed their points into Mary's skin, injecting venom from the sacs.

Mary gasped. A stream of bubbles escaped from her mouth. Searing pain burned over her arms and torso. She kicked toward the surface, tearing at the stinging things adhering to her skin.

Gil, leaning back on his elbows and staring impassively through his tortoiseshell sunglasses, watched her graceful dive. The surface erupted with foam. He sat up. It was Mary, her arms flailing wildly. Her screams carried across the calm water.

He knew immediately what had happened.

"I'm coming!" he shouted, launching himself off his towel. He would be the heroic husband, vindicating himself for the caution that she had so derided. He flung off his $375 sunglasses and charged into the water. Then he suddenly stopped.

Would it sting him too?

She was about 50 feet away, in water that was probably just over her head, struggling to put her feet on the bottom. Where was the jellyfish? It could be anywhere. How long were its tentacles? He had no idea.

"Where is it?" he shouted.

It was as if Gil, standing paralyzed in knee-deep water near shore, had also been injected with the jellyfish's powerful, stunning venom. He looked up and down the beach for someone who might help, but they'd chosen this beach precisely because no one was here.

"Can you swim to me?" he shouted.

She raised an arm helplessly, and he could see the whiplashlike welts already spiraling along it as she cried and gagged in the water.

No one knows exactly what's in the venom of Chironex fleckeri that makes it so potent. Research has been stymied in part because the venom is susceptible to heat, making it difficult to analyze without chemically altering it. Scientists believe that the venom, made up of proteinlike substances, contains three major toxins: one that damages skin, one that affects the blood, and another that works, sometimes fatally, on the heart and other organs. The incredible skin pain, some have speculated, could be due in part to a compound called 5-hydroxytryptamine, found in the tentacles of many types of jellyfish as well as in bees and stinging nettles. It has been estimated that Chironex venom enters the circulatory system of a healthy person within 20 seconds of the sting, as the tiny tubules inject thousands of doses of venom directly into the capillaries just beneath the skin. This is unlike snakebite, in which the snake's fangs leave a few large deposits of venom that take hours to be absorbed into the tissue.

Mary's "fight or flight" response—which might be useful if she were fending off a shark attack—had sent surges of adrenaline through her body, boosting her heart rate from 80 beats per minute to its maximum exertional rate of 180. Her rapidly contracting muscle tissues, which demanded oxygen, triggered a rush of blood from her heart and lungs to her limbs and back again. The needs of her muscles, induced by her panic, helped carry the venom from the capillaries beneath her skin straight to her heart.

The venom quickly did its work. Something in the venom—a cardiotoxin, as it's known—began to wreak havoc on her heart's electrical system, causing a big upsurge in the number of positively charged calcium ions entering the cardiac muscle cells. An excess of calcium ions inside these cells causes spasms in the carefully timed contractions of the heart, a bit like throwing a jug of water onto the circuitry of an electric motor.

Mary's heart abruptly lost its rhythm. The ventricles, the heart's two high-pressure pumping chambers, normally contract smoothly from the bottom up, squeezing blood out the top. But theChironex venom triggered chaotic contractions originating somewhere in the middle of the ventricular walls and firing at an arrhythmic pace of 240 beats per minute. Instead of a powerful, coordinated stroke pushing out three ounces of blood with every beat, her heart dribbled less than a twentieth of that amount. Her blood pressure plummeted. The blood flow to her brain slowed to a trickle. The bright circle of sunlight and seawater and pain faded to a twilight pool.

Her head dropped in the water, bobbed up, dropped again. She wasn't screaming now, though her lesioned arms still waved vaguely, keeping her afloat.

"Mary! Mary!" Gil shouted.

She gave no indication that she heard him.

Gil had never seen anyone die before, but it was perfectly clear to him that she was almost dead. He knew he'd replay this moment for the rest of his life: Mary floating face down, arms stirring dully, hair gently fanning out on the water, while he stood frozen and simply... watched.

He moved one foot forward across the sandy bottom. Then the other. Suddenly he was churning through the water toward her, no longer caring what happened to him, as long as he acted. Thigh-deep...waist-deep...chest-deep. She lay floating only ten feet away. He shuffled two steps closer. He stretched out his right arm, reaching up over the surface of the water, above any stray tentacles. He touched her left hand, feeling the sticky softness of the strands that were wrapped around it. He pulled away. Oddly, he felt no stinging. Were they somehow spent?

They were not. He felt no sting because the tiny venom-filled tubes launched by the nematocysts didn't have enough power to penetrate the thick skin of his palm. The hair on the back of his hand also acted as a barrier to their penetration. Women and children, who have less hair, smaller bodies, and more tender skin than men, are therefore more susceptible to Chironex stings. Even a thin layer of nylon can repel stings, as discovered by Australian surfers and lifeguards, who pull pantyhose over their legs and torsos when swimming in jellyfish-infested waters.

Gil reached out again and gripped her left wrist, between tentacles. Adrenaline pumping, he dragged her 122 pounds through the shallow water and pulled her up onto the beach.

He dropped to his knees beside her. Strands of tentacle clung to her torso, etching purplish-brown weals on her skin. If she survived, the tissue could die, and she might be scarred for life with the whorled markings, as if tattooed by a heap of rope.

Gil remembered the sign posted at the beach near town: "Flood sting with vinegar." Dousing tentacles with vinegar prevents the nematocysts from firing. Gil had noticed the jugs placed at regular intervals along that beach, but there was no vinegar here. He grabbed his towel and wrapped it around his right hand, plucking the tentacle fragments from Mary's skin.

The severity of a Chironex sting depends largely on how much tentacle has made contact with the victim; about six feet is considered the minimum for a lethal dose in an adult human. By some estimates, a full-grownChironex contains enough venom in all of its tentacles to kill up to 60 people.

About 15 feet of tentacle had made contact with Mary's skin. Gil rolled her over. He watched her sand-covered abdomen for the rise and fall of breathing. Nothing.

Years earlier Gil had taken a course in CPR and then later, a refresher course before their trip to the Bahamas. He pinched Mary's nostrils, placed his open mouth over hers, and exhaled once. Her chest rose slightly as the air went in, then fell. But what about her heart? He felt for a pulse in her neck, holding his trembling fingers as still as he could. Nothing. Her heart was still in wild spasms, pumping at over 200 beats per minute, but her blood pressure was too low to register a pulse.

Gil had to become both her heart and her lungs, at least long enough for her body to recover from the venom's shock and regain some of its equilibrium. This was not a simple task for one person to perform. He'd have to work quickly. Laying his hands on top of her breastbone, he compressed her sternum, then released and shoved again. After the first 15 thrusts he gave her two more quick breaths, followed by another set of 15 compressions. His rapid, steady thrusts kept blood moving up her carotid arteries and into her brain at one-third its normal flow—not a lot, but enough to deliver the crucial supply of oxygen to the brain's starving tissues.

Thirty seconds passed. A minute. The pumping action he forced on her heart began to wash away the poisoned blood, dispersing it throughout her body.

Another 40 seconds passed. Sweat flew from Gil's face and arms. He didn't know how long he could keep this up.

Embedded in the roof of the right atrium is the heart's own pacemaker, a bundle of muscle fibers called the sinoatrial node that generates its own electrical charges. As it fired, it tried to send impulses through Mary's heart like a wave, ordering contractions of the exact proper muscle tissue in the exact proper sequence. But tiny holes in her cell membranes remained opened from the venom's effect, and the calcium ions passed too easily into the cells, keeping the muscle in spasms. For her heart to resume its normal rhythm through CPR, Gil would have to keep up the chest compressions long enough for the venom to degrade, 30 to 40 minutes.

The second minute passed. Gil stopped again to probe for a pulse. He felt something. But what? He placed his hands on her sternum and resumed the frenzied pace of chest compressions punctuated by breathing. Another minute. He was panting so hard now he had to pause to catch his breath. It seemed that he'd been working over her for a very long time. Was this futile? He felt again for her pulse, trying to understand what he sensed in his fingertips. Then he heard a small gasp. He looked at her chest. Another gasp. He saw her rib cage fall slightly.

He thrust his ear to her chest. He could hear something. Was it thelub-dub of his own thumping heart or the sound of her heart valves opening and closing as they should?

"Keep going, Mary!" he shouted.

He lifted his head from her chest. Now her breath came in small gasps.

"That's right, breathe, Mary, breathe!"

Her limbs stirred, brushing against the sand as if she remembered, deep in her subconscious, that she was supposed to swim. Her head lolled back and forth. Her eyes were still closed. Gil knew he'd have to pick her up in his arms and haul her to the car, then jounce over the rutted beach road to the highway to town. They'd arrive in 20 minutes, and the hospital would surely have a ready supply of the box jellyfish antivenin. Quickly administered, the antivenin (made from the blood of a sheep that's been immunized to the venom) does much to ameliorate the jellyfish's sting. She could relapse—Chironex fleckeri victims sometimes show a brief improvement and rising blood pressure before suddenly expiring—though Gil didn't know this. They'd put her on a respirator if she needed it, and give her medicine for the pain that would return once she woke up.

What would happen then? he wondered. What would they say to each other? Would she in some way be a different person, not pushing him always? And would he be different, not hanging back? Could a brush against this tentacled creature that had no brain, no sense of good and evil, change a life, two lives, a marriage?

He jumped up. He ran to the pile of clothes they'd shed on the beach and grabbed his daypack with the car keys in it. He knelt beside her again.

"Come on, Mary, come on!"

He allowed himself to wish now, to hope. Gil knew that he'd done everything for her that was humanly possible, far more than he would have ever guessed he could do. As her chest continued to rise and fall, tears began to fill his eyes.

Peter Stark's Last Breath: the Limits of Human Endurance, from which this article was excerpted, was published in October 2001 by Ballantine. His book Astoria: John Jacob Astor and Thomas Jefferson’s Lost Pacific Empire: A Story of Wealth, Ambition, and Survival will be published in March 2014 by Ecco.

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