Remember, it isn't the shark you see that's gonna get you," said Doc Anes. Doc's cherubic face was in shadow from his wide-brimmed hat, but I could sense his mischievous grin as he steadied one of the two cages at the stern of the boat. The water was rough – a northeast wind had been sweeping down the island for a couple of days, depriving this bay of the shelter it normally gets from the prevailing northwesterlies – and the cages bobbed and clanked and slammed into each other. Built like a barrel of nails, with two Smithfield hams for forearms and a pair of fireplugs for legs, Doc put a foot on one cage and forced it to submit to his will.

He nodded to me, and I took a step toward the ladder that led down into the cage. Looking like Shrek in a dress, I had on a lovely new trilaminate drysuit (accented with zippers rugged enough for a body bag), 40 pounds of lead that hung from my shoulders by chic suspenders, a sleek black neoprene hood, and a snazzy yellow face mask.

The dark blue water sparkled with whitecaps – a scene of perfect harmony, except for the smear of blood and fish guts and oil that spread behind the boat and drifted with the tide. But somewhere out there, cruising, agitated, tantalized by the rich scent lacing the water, was the largest carnivorous fish in the world, one of the few predators left on the planet that poses a genuine threat to man in an environment in which he chooses to go. Somewhere out there – not far, probably close by, perhaps directly under the boat – was a great white shark. At least one, maybe more.

I descended the ladder, and right away I was grateful for the drysuit. The water was frigid, between 61 and 63 degrees Fahrenheit, but I wasn't cold. As soon as I ducked below the surface, I located the regulator mouthpiece and air hose that had been thoughtfully clipped at eye level to one of the aluminum bars of the cage. I purged the mouthpiece, popped it in, and drew a long, easy breath while my wife Wendy was making her way into my cage.

The visibility was terrible, no more than 10 feet; there was no way we'd be able to see a shark, even one as big as –

There she was, hovering perhaps six feet away, as if she had been waiting for us. I held my breath. She was enormous, at least as long as a schoolbus. She turned and showed me her flank, and now "schoolbus" wasn't adequate. Locomotive. That's how big she was, as big as a locomotive, and . . . 

Hold on, I told myself, get a grip, remember: Water magnifies everything by roughly a third. I tried to do a proper measurement. The cages were each 10 feet long. From nose to tail, the shark extended the entire length of my cage and halfway along the next, so she was 15 feet long, roughly. What rendered her gargantuan was her girth. She looked as big around as a midsize SUV. She had to weigh two tons. Four thousand pounds. The mother of all fishes.

With a couple of easy sweeps of her tail, she turned toward the cage and turned again, moseying on by. Rays of sunlight stippled her back with blues and grays; her belly, even in shadow, was ghostly white. She showed us her lower jaw, studded with snaggly gripping teeth. Her upper jaw was curled under, concealing the rows of triangular cutting teeth and giving her the look of a toothless old codger.

But she was no codger. This lady was in the prime of her life. She was gorgeous, physical perfection, an animal so precisely tuned to her environment that it had not been necessary for her kind to evolve significantly in millions upon millions of years. She had been mistress of her world for eons.

Before her majesty, I could feel nothing but puny.

Immediately, words from Henry Beston's celebrated The Outermost House sprang into my mind. (If that seems preposterous, so be it; these words are engraved on my frontal cortex, and they leap to the fore whenever I'm privileged to be in the company of one of nature's magnificent giants.)

"We need another and a wiser and perhaps a more mystical concept of animals. . . . We patronize them for their incompleteness, for their tragic fate of having taken form so far below ourselves. And therein we err, and greatly err. For the animals shall not be measured by man. In a world older and more complete than ours they move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear. They are not brethren, they are not underlings; they are other nations, caught with ourselves in the net of life and time, fellow prisoners of the splendour and travail of the earth."The shark gazed at Wendy and me with one eye, solid black, utterly without expression, the eye described by Peter Matthiessen in Blue Meridian as "impenetrable and empty as the eye of God." As the shark moved on, Wendy turned to me and made a "Yes!" gesture with her fist. She was ecstatic; this was her first genuine experience with a white shark. We had been on scores of dive trips in our 40 years of marriage, and she'd swum with sharks of many kinds and sizes, but never had she gone eyeball-to-eyeball with a great white. And here, on the first dive of the first day, it had already paid off.

As many times as I've seen white sharks underwater, I'm never bored or blasé. Among fish, among sharks, among predators, they're unique, not only in appearance but in behavior as well. They move with a serenity born of invulnerability, with an inexorable confidence that no predator can harm them and no prey elude them. They don't circle tentatively like other sharks, appraising potential danger; they move straight in and then decide if what they've approached is worth biting.

They also feel good. As beautifully supple as they appear, they're as hard as steel. I wanted to touch the shark's tail fin, to feel once again that too too solid flesh. I put my arm out through a camera port, a gap in the bars on the cage, and reached with my hand for the disappearing tail, and –

Suddenly I was thrown back against the far bars of the cage, knocked aside by a pressure wave. I saw a flash of gray and white streak up from the gloom below, pass within inches of my outstretched arm and lunge at one of the tuna heads that dangled from the boat as bait.

"It's not the shark you see that's gonna get you. . . ."

There were 16 of us, plus a crew of eight, aboard the Horizon, a 75-foot-long dive boat, and we had traveled some 220 miles southwest of San Diego to this isolated rock called Isla Guadalupe. The island itself is a Mexican seal and sea lion sanctuary, though the waters around it are not, and it is all but uninhabited: A tiny colony of artisanal fisherfolk live near the southern end, and they eke out a living from the migrating pelagic fish, mostly varieties of tunas.

Until a few years ago nobody had bothered with Guadalupe for anything but fishing, but then some sport fishermen wondered why most of the big fish they were catching arrived at the back of their boats bitten in half, and from that question a tiny tourist industry was born: great white shark watching. Divers are a questing lot, always searching for new places to go and new animals to see – but for a price. If lions and elephants are among the five or six great prizes for terrestrial photographers, great white sharks are the holy grail for underwater photographers. For decades, however, they were out of range for all but the certifiably wealthy. Accepted wisdom held that great whites could be photographed only in South Australia, and a trip down and back, plus a week on a boat, could cost $7,500 or more.

With the demise of apartheid, South Africa opened up, and great whites were discovered there along the southeast coast. (Discovered by tourists, that is; locals, of course, had known about them forever.) You could venture offshore in a small boat for a single day and see great whites for a very reasonable $100. Still, there was the prohibitive transatlantic and transhemispheric airfare.

Then the word began to spread about Isla Guadalupe, a mere 22-hour boat ride from the U.S. West Coast. For three months of the year – September, October, and November – groups of white sharks visit Guadalupe. (By groups, incidentally, I mean a handful, perhaps two dozen in total. Great whites are solitary by nature and scarce worldwide in the best of times, and these are far from the best of times.) No one is quite sure why they arrive on such a tight schedule or in such a regimented way. First come the young, pre-adult males, 10 or 12 feet long, then the larger, adult males, then the younger females and, finally, in late November, the big mamas: adult, breeding-age females, up to 21 feet long and weighing up to six thousand pounds.

According to Jessie Harper, a tough and resourceful researcher who doubles as a shark wrangler, deck hand, and all-around seaman, the same sharks show up every year, and one of her ongoing projects is to develop a photo archive that lists and identifies individuals by scars, color patterns, and the shape and silhouette of dorsal and tail fins. A curious contradiction inherent in Jessie's work is that despite the popularity of great white sharks, despite the waiting lists of divers, scientists, and photographers eager to join white shark expeditions, any scrap of information Jessie or her colleagues can gather will probably be new and may possibly be of importance. Thirty years after the release of the movie 'Jaws,' and despite countless studies by countless professionals, very, very little is known about great whites.

No one knows for certain how many there are worldwide, how long they live, how old they are when they reach breeding age, how many pups they can have, how far they travel. No one knows whether their diet changes during the course of their lives or if it is possible, as is now being conjectured, that great whites learn from one another, that adults pass information to their young – which, given an odds-on opportunity, they would otherwise gladly eat.

A stable population of great white sharks such as the one at Guadalupe is, therefore, a treasure for researchers.

For the four days we were there, Guadalupe proved to be Fantasy Island. Never had I seen so many white sharks – 17 individuals in all. Never had I seen sharks that were so dependable: In good weather and bad, in clear water and lousy, the sharks were there, crisscrossing the chum slick, charging at the tuna-head baits that trailed behind the boat, performing magnificently for the cameras that protruded from every gap in the bars of both cages.

As predicted, they were all females, all big: The largest was 16 feet long, which meant that underwater she appeared 20 feet long and fully capable of swallowing any one of us whole. Twice, due to the swinging of the boat at anchor and the movement of the cages, the hanging baits dangled directly in front of the camera ports, and twice sharks lunged for the bait, missed, and jammed their heads inside the cage. Unable to swim backward, panicked by the sudden imprisonment, they thrashed and rolled and slammed the cages against the boat, and for a moment there seemed a fair chance that one might tear a cage away from the boat and take its four occupants out into the open, where it might . . . well, who knew?

But it never happened. The crew, under Doc's supervision, maneuvered cages, sharks, and people with great skill. The sharks swam free, and everyone returned from every dive unscratched, unscathed, and untraumatized . . . 

 . . . until supper on the final evening, just before we began the run back to San Diego.

Someone said, "Did you see that new one today, the one with the big breeding scars on her gills?"

Someone else said, "What about her?"

"She had a hook and six feet of wire leader sticking out of her mouth."

There was silence until one of the crew said, "The other day, some of the local fishermen came by and said they'd seen a big sportfishing boat hooked into two white sharks. They said they went up to the boat in their panga [boat] and asked the guys to unhook, and the guys fucked 'em off. So they cut the lines. Then they took off, figuring the guys probably had guns aboard."

"Did they report the boat?" I asked. "Call the Mexican coast guard?"

"The name, home port, and numbers had been painted over, but it wouldn't have done any good anyway."

"Why not?"

"Fishing for white sharks isn't illegal," he said. "Not in Mexico. It's not popular, nobody likes it, but it's done. A really huge jaw can bring a guy 10,000 bucks."

With that, Fantasy Island suddenly became the island of the doomed. These great white sharks, these exquisite examples of natural perfection, might be wiped out within a year or two. And not just this population. Despite a few very important recent developments in conservation, worldwide the odds remain stacked against the survival of sharks in general and of great white sharks in particular. Having endured – triumphed! – through uncountable, unknowable cataclysms to reign unchallenged over the oceanic food chain for scores of millions of years, these hardy apex predators may come to terminal grief in the greedy grip of the most savage, random, destructive slaughterer of all: us.Statistics are dry, lifeless, and, especially when applied to fish and other animals whose populations are (or have been) vast, necessarily approximate. Unlike dolphins and other marine mammals, which must come to the surface to breathe, sharks never have to surface and thus are impossible to count accurately. Still, the current statistics about sharks are appalling.

It is estimated – and I'll say this only once because nearly everything about sharks is estimated – that roughly 100 million sharks are killed every year by fishermen. (By contrast, fewer than a dozen humans are killed by sharks in an average year.) Most are killed intentionally, but millions die as what is euphemistically known as "bycatch," which means that they're killed while fishermen are aiming to kill something else. In the North Atlantic alone, populations of sharks and other large pelagic fish (including tunas and billfish) have been reduced by 90 percent over the past 20 years.

Ninety percent! When I grew up in Nantucket in the 1950s, the sea on a calm day appeared to be carpeted by blue sharks. The dark triangles of their dorsal fins sliced through the surface of the water, touching ancient nerves and causing hackle-raising nightmares in all who saw them. Nowadays you're about as likely to see a blue shark around Nantucket as you are a unicorn.

Long-lining and drift netting are two of the worst, most damaging methods used in commercial fishing. A longline is exactly what its name implies: a line that is long, up to 80 miles long, containing thousands upon thousands of baited hooks that kill literally everything: birds, turtles, seals, dolphins, and sharks, in addition to the tunas, billfish, jacks, and other food fish that are their targets.

Drift nets, too, do what the words imply, but the innocuousness of the name disguises the carnage they wreak. Enormous nets made of strong, thin, nearly invisible plastic filament are cast loose from fishing boats and, buoyed by floats, set adrift with the currents to snare what they can. They catch everything too big to pass through their small mesh. The worst damage they do is when they're lost, when, through carelessness or violent weather, their floats break away and the net sinks below the surface, to drift aimlessly and kill endlessly.

The decline in most fisheries can be attributed to a simple fact: There are too many people in too many big, fast, and efficient boats, equipped with too much brilliantly effective new technology for locating and catching fish, all pursuing too few fish.

We're wiping out the oceans.

With sharks, however, there's an extra, odious industry that is putting them in potentially terminal danger. It is called (again the euphemism) finning, and its object is (how innocent!) soup. No, that's not true. Its eventual product is soup; its object is status.

Shark-fin soup has for generations been an expensive delicacy in Asia. To my taste, it's stringy and slimy and mucusy and tasteless – but savor isn't the point of shark-fin soup. The point of serving it, and eating it is to show off. In a fancy restaurant, it can cost upward of $100 a bowl, and to serve it at a large affair, like a wedding, is to make a loud statement about one's wealth and success.

Until relatively recently, shark fins were acquired the old-fashioned way: Artisanal fishermen caught sharks on hand lines, cut off the fins, and used the rest of the sharks' carcasses for everything from oil to cosmetics, folk medicines, abrasives, and, of course, food. Sharks died, yes, but not too many, and the entire animal was put to use. The fishery was sustainable.

Nowadays, however, the rise of wealthy, active, mobile middle classes in many Asian countries – especially China – has driven the demand for such pricey items as shark-fin soup into a frenzy, and their value has soared. No longer is it worth keeping or processing entire sharks; the carcasses take up space that could be better filled with fins alone. And so, huge factory ships haul in their 50- and 60-mile-long lines, cut the sharks away from the hooks, slash every fin off every shark, and toss the still-living animals back into the sea, there to fall helplessly to the bottom and asphyxiate.

Although cruelty is certainly an issue, it isn't anywhere near as important as survival of the species. Depending on whose statistics one consults, of the more than 350 known species of shark, roughly one quarter of them are, to one degree or another, endangered. A handful, including the great white shark, may ultimately face total biological extinction, meaning they will go the way of the dodo and vanish from the face of the earth.

Sharks' own biology is among their biggest problems. Most species are slow-growing. They breed relatively late in life – white sharks aren't ready to breed until they're at least 12 years old – and they produce very few young. (Some species produce dozens, but the offspring have been known to eat one another in utero.) And these days, living for 12 years is an increasingly rare accomplishment for a shark. Once depleted, shark populations take years to recover, and with pressure on them always increasing, most simply don't. According to a report from the Ocean Conservancy, "There is no evidence of any white shark population recovery, even in areas where the species has been protected for many years."

The shark-finning industry was once limited to Asia, but demand has caused it to spread all over the world, from Australia to Burma to South and Central America. I've dived in wildlife preserves off the coasts of Mexico, Ecuador, and Costa Rica and seen the bottom littered with the corpses of finless sharks. I've seen boats full of poachers finning sharks with no concern for who was watching, and no wonder: When we reported them to the local authorities, we were told to mind our own business.

Sharks have never been very high on humans' lists of priorities, and the reasons are obvious. Unlike marine mammals such as dolphins, which nurse their young and chatter among themselves and appear sweet and charming and very . . . well . . . human, sharks have always had a bad rap. They look mean, and they have a reputation for killing people. It's hard to build a constituency for an animal that may decide to eat you.

On another level, popular ignorance about sharks has been a reflection of our stupendous ignorance about the ocean world. More than 70 percent of the planet is covered in water; more than 95 percent of all the living things on earth live in the ocean; there are mountains under the sea taller than Mount Everest and canyons miles deeper than any on land. And yet we persist in ignoring the oceans in favor of exploring the moon and Mars. We have studied less than 5 percent of our water planet, and we've actually visited less than 1 percent of that 5 percent.

It's no wonder we haven't found the time or the interest to learn much about sharks. But over the past few months there have been a couple of very positive developments. In October of 2004, after years of wrangling, the UN Convention on International Trade in Endangered Species (CITES) placed great white sharks on its Appendix II list, which prohibits trade unless a country can demonstrate that it won't be detrimental to the species. In the case of great whites, which are valued for fins, jaws, skin, and meat, such proof is impossible. The practical effect of the listing will be difficult to assess, but its symbolic effect is important: It shows that the international community at last recognizes that certain species of shark are in danger and, more significantly, are worth saving.

A month later, the International Commission for the Conservation of Atlantic Tuna (ICCAT) banned all shark finning. No one knows if the ban will be enforceable, but at least it's on the books.

Certainly the public's attitude toward sharks has changed over the years, from hostility in the old days, to fascination during the 'Jaws' craze in the 1970s, to interest and, evidently, growing affection nowadays. When a 15-foot female white shark was marooned in a pond in Massachusetts after a storm last fall, the public was galvanized – not to harpoon her, as would have been the cry years ago, but to save her. The drama was big news across the country for a week, until wildlife officers finally found a way to coax her over a shallow bar to freedom.

And when a very young white shark, only four feet long and weighing 60 pounds, was saved from a California fisherman's net and taken to the Monterey Bay Aquarium last August, attendance at the aquarium increased 50 percent overnight. At first officials feared that she would refuse to eat or would harm herself by blundering into the sides of the million-gallon tank, but she has thrived. People have come from all over the world to see her, and their faces and their questions express awe, rapture, and affection.

I like to think that after thousands of years and hundreds of generations of fearing sharks and hating them and wanting to kill them, perhaps we're beginning to appreciate them for the magnificent animals they are. As Harvard sociobiologist E.O. Wilson has pointed out, whether we know it or not, we humans have a profound emotional stake in the continued existence of sharks.

"We don't just fear our predators," he wrote, "we're transfixed by them, prone to weave stories and fables and to chatter endlessly about them, because fascination breeds preparedness and preparedness, survival. In a deeply tribal sense, we love our monsters."