As the sea churned all around us, we leaned over the edge of the boat to get a better look. Dozens of dorsal fins cruised here and there; somewhat menacing half-moon tails thrashed about; and, now and then, a colossal gaping mouth breached the water's surface. The creatures' size was daunting; their beauty captivating. Scores of white spots and faint stripes decorated their gray-blue backs—patterns that evoked ribbons and specks of light weaving and glinting on a rippling sea.

“Jump!” our guide Jose shouted. Donning snorkel masks and fins, my younger brother Brandon and I leapt into the warm waters surrounding Mexico’s Yucatán Peninsula. At first, I saw nothing but turquoise. Then a looming silhouette, a fin, an eye. With startling speed a bus-size shark swam past us, only a few feet away. A pair of young sharks approached from a different direction. And another. Their enormous gills flexed and flapped. Ghostly suckerfish clung to their sides. All these sharks—between 500 and 800 in total—had gathered for a seasonal buffet. Fortunately, we were not on the menu. In fact, the sharks seemed largely indifferent to our presence, hardly altering course to avoid colliding with us. Given their size, we were the ones who scrambled to get out of the way.

Stretching more than nine meters and weighing around 10 metric tons, whale sharks are the largest fish in the world. They are also the giant polka-dotted Roombas of the ocean. Although they have more than 300 rows of tiny teeth in their mouths they do not seem to use them. Like basking sharks and megamouth sharks, whale sharks are filter feeders. By opening and closing their mouths to create suction or swimming with their mouths hanging open—as though in a perpetual state of shock—whale sharks direct huge volumes of water across 20 fleshy sievelike structures in their throats. As the water escapes through the gills, those spongy filter pads catch fish eggs and billions of microscopic drifting animals and plants known as plankton. Sometimes the sharks dine on krill and squid; they've even been filmed vacuuming silverside fish out of fishermen’s nets. Eventually all that edible flotsam rolls into a big briny bolus that the shark swallows with a kind of shudder.

Twenty years ago, scientists did not know much about what whale sharks ate, where they spent their time or how they reproduced. Historically, seeing a whale shark in the wild was a rare experience, even for veteran divers. Jacques Cousteau reportedly encountered only two whale sharks his whole life. Up until the 1990s most shark biologists had no idea that whale sharks routinely convene in the dozens and hundreds in coastal regions. Discovering such congregations—first in Australia and later in the Yucatán—allowed researchers to locate, tag and observe large numbers of the animal season after season. Studies in the last 15 years have greatly improved researchers’ understanding of these giant fish and overturned previous misconceptions. At the same time, knowledge of the fish’s seasonal smorgasbords has created a new form of ecotourism, the full consequences of which remain unclear. When properly managed, tourism seems to benefit the sharks. Some scientists worry, however, that shark tourism in Mexico is growing too large too fast, threatening to usurp one of the fish's primary feeding grounds.

Just about every year since 1994 Robert Hueter of the Mote Marine Laboratory in Sarasota, Fla., has traveled to Quintana Roo in the Yucatán to study sharks. He focused almost exclusively on the blacktip shark until a fateful conversation in 2002, when a Mexican fisherman casually mentioned that shortly after Hueter and his team left each year hundreds of whale sharks gathered nearby. “Come again?” Hueter said. In more than 40 years as a shark biologist he had seldom seen or studied whale sharks. Researchers already knew that the fish assembled in Australia but Hueter had never heard of them meeting in Quintana Roo—certainly not in such large numbers. Apparently, local fishermen had known about these gatherings for many generations but word had never reached the scientific community at large.

Scientists began to survey the legions of whale sharks near Quintana Roo from the sky and sea, pinning the fish with ID tags and small electronic devices that would record temperature and depth and measure daylight duration to estimate latitude and longitude before beaming all this information to a satellite. Meanwhile researchers in Australia had been tagging whale sharks as well. There's still a lot to learn, but studies tracking whale sharks have already divulged a few secrets.

Researchers have established that whale sharks migrate thousands of miles annually, uniting in more than a dozen spots around the world including the Yucatán, Madagascar, Australia and the Philippines to gorge on plankton, krill and fish eggs. For reasons that remain unclear, most of the sharks in these collectives are young males. Hueter suspects that female whale sharks also complete long journeys to give birth. In 1995 Taiwanese fishermen harpooned an 11-meter whale shark with 300 fetuses inside her two uteruses, confirming that whale sharks give birth to live young, rather than laying eggs encased in leathery pouches like some shark species. Scientists also learned that whale sharks are late bloomers: they may live to 100 and do not reach sexual maturity until sometime between 18 and 30 years. But where the fish mate and gave birth remained a mystery. To date, no one has observed whale sharks breeding in the wild. Between August 2007 and January 2008, however, one of Hueter’s satellite tags tracked a female with a swollen belly traveling about 7,200 kilometers from the Yucatán to a region of the mid-Atlantic just below the equator. There, Hueter proposes, she gave birth. Young sharks would be much less vulnerable to predators in the open ocean than in highly trafficked coastal areas. Recent research has also revealed that whale sharks regularly dive into deep, cold water. In 2008 Hueter and Eric Hoffmayer of the National Marine Fisheries Service recorded a whale shark diving a record 1,928 meters in the Gulf of Mexico. The reigning hypothesis is that by gently gliding toward the ocean’s bottom while still moving forward, sharks manage to continue their journey, cool off and conserve energy all at once.

Not only are whale sharks largest fish in the ocean, they are the heaviest as well, rivaling Tyrannosaurus rex and other giant dinosaurs in size and weight. As Hueter and his Mote colleague John P. Tyminski have written, whale sharks are probably the largest animal on Earth that people can approach in the wild without being in any real danger. (Anyone concerned by the fish's cavernous hoovering mouths will be glad to hear that the chances of reliving Jonah's experiences are slim to none. A 2011 Daily Mail story claims that one diver was almost swallowed by a whale shark, offering photographic evidence of the supposed close encounter; photographer and documentarian Mauricio Handler of Maine, who took the photos, says he and his friend were in no danger whatsoever. Besides a whale shark's esophagus is too small to accomodate a human.) Around the same time that Hueter first learned of whale sharks off Quintana Roo, Willy Sabatina and other entrepreneurial fishermen on nearby Isla Holbox began to experiment with organized whale shark tours, which were already popular in Australia. They realized people would pay handsomely for the opportunity to see and swim with the gorgeous, gentle giants. The following year Mexico’s National Commission to Protect Natural Areas (CONANP) established the Domino Project, a collaborative research and conservation program intended to study whale shark biology and protect the creatures as whale shark tourism became increasingly popular.

Whale shark ecotourism is now a global phenomenon, drawing people to Mexico, Thailand, South Africa, Mozambique, Honduras, the Maldives and Ningaloo Reef in Australia, among other locales. In some ways, whale shark tours are an example of well-intentioned, well-executed ecotourism that arguably benefits both the animals and local communities. Fishermen and conservationists in Ningaloo, Belize and Mexico have established codes of conduct to protect the sharks: touching the fish is not allowed, for example, and only so many snorkelers are permitted in the water at any one time. Whale shark tours can discourage fishing and finning, superseding them as a more profitable source of income. The Philippines shut down its whale shark fishery when fishermen started earning far more money as tour guides. And the popularity of whale sharks has transformed Isla Holbox and neighboring Isla Mujeres from relatively sleepy fishing towns into bustling tourist hotspots where people from the U.S., Europe, Australia and Asia set themselves up in air-conditioned hotel rooms, order margaritas by the pool, drive around in golf carts and pay people on the islands to take them sport fishing, snorkeling and scuba diving. In turn, such revenue can spur governments to protect the animals that attract so many visitors. Encouragingly, the Western Australia Department of Environment and Conservation reports that (pdf), overall, ecotourism does not seem to hurt local whale sharks. Mark Meekan of the Australian Institute of Marine Science is convinced that ecotourism has helped safeguard whale sharks in his country.

Whether shark tourism is similarly beneficial in the Yucatán is much less clear. In part because of their proximity to the internationally renowned resort town Cancún, Isla Holbox and Isla Mujeres are home to the fastest growing whale shark tourism industries in the world. Whereas Gladden Spit in Belize and Ningaloo Reef in Australia have only 15 to 30 licensed tour boats each, the Yucatán has 240 licensed tour boats as of 2012, each of which can carry around 8 to 10 people. Hueter, Tyminski and other researchers argue that ecotourism in the Yucatán and elsewhere harms whale sharks in known and unknown ways. In Australia, where whale shark tours have been around longer than anywhere else, officials routinely fly over the few tour boats in small planes to make sure everyone is following the rules. Mexico's CONANP, however, does not have enough resources or authority to properly enforce their codes of conduct. Instead, they essentially rely on the honor system: tour guides are supposed to report one another if they see inappropriate behavior. In all likelihood, rules are sometimes bent and broken, even in places where ecotourism has brought new respect for the whale shark. In the Philippines some guides have taken to hand-feeding whale sharks to keep them near boats; others in Kenya have proposed corralling the fish into lagoons to guarantee tourists an encounter.

Research conducted in Australia shows that between the mid-1990s and today the average body size of local whale sharks has shrunk by two meters and the total population has declined. Hueter and a few other scientists say ecotourism may be to blame, at least in part. They speculate that snooping snorkelers and hovering boats inhibit sharks from feeding and discourage them from returning to their favored feeding sites. Although the sharks I encountered near Isla Mujeres did not seem to mind all the human visitors, they are known to dive when startled or touched on the head. And I have to admit that the whole situation felt too claustrophobic: so many sharks, people and boats in a rather small patch of ocean. Because whale sharks spend a lot of time filter feeding at the surface, they are vulnerable to potentially fatal collisions with boats and their propellers. The surging growth of whale shark tourism in the Yucatán would only exacerbate these potential problems.

Meekan and other scientists argue that the link between ecotourism and whale shark population decline is tenuous and contradicted by direct evidence. "I am very firmly of the opinion that the declines have nothing to do with tourism," Meekan says. "We can individually identify sharks by their stripe patterns and, based on our data, we can say that animals that have swum with many tourists are no less likely to return to Ningaloo than whale sharks that did not swim with many tourists." The real threat to whale sharks, Meekan maintains, and the likely reason for Australia's shrinking whale shark population, is fishing. In the Gulf of Mexico and Caribbean people never heavily fished whale sharks because they were not considered good eating. Taiwan and other parts of Asia, however, have prized whale shark flesh and fins. Although Taiwan, India and other countries have closed their commercial fisheries, China's appetite for whale shark is growing. Whale shark meat is so thick and firm that people in Asia have dubbed it "tofu shark." Some restaurants want the fish's huge fins not for the shark fin soup they sell, but rather to hang in windows and on doors as billboards. Whale sharks have also been found in the Philippines and Maldives impaled with spears or missing all their fins.

Definitively disentangling the influence of tourism from fishing will require more reliable long-term data. For now, it may be safest to assume that they both take their toll, at least when ecotourism is new and relatively unregulated. "The operation in Yucatán in its infancy," Meekan says. "I'm fairly confident that with time everyone will realize that protecting the sharks is in their best interest. That has certainly been the pattern."

The Kiswahili word for whale shark is papa shillingi, meaning “shark covered in shillings.” A Kenyan legend says that soon after creating the whale shark, God and his angels scattered gold and silver coins across the fish’s back in honor of its beauty. In a sense, we’re still throwing money at whale sharks. The question is whether, instead of entertaining ourselves at the expense of these magnificent animals, we can satisfy our curiosity and honor them at the same time.