OFF THE YUCATAN COAST - I'm hanging in a bottomless abyss of blue-green, peering through a mask, when a shadowy shape practically the size of an 18-wheeler glides into frame.
A 40-foot fish swims past, its mouth so wide I can see down its throat as it scoops up millions of nearly invisible fish eggs. I'm awestruck. As a scuba diver with more than 300 dives under my belt, I've long dreamed of glimpsing a single whale shark. Today, I'm surrounded by dozens of the giant plankton-eaters.
About a decade ago, Yucatan fishermen discovered whale sharks massing in the warm waters 20 miles off the coast. The sharks, which they call "domino" or "checkerboard" fish, were drawn here by the hundreds during the summer and into September to gulp the pinhead-size eggs of spawning little tunny, a type of tuna.
Big, graceful, and toothless, the sharks soon became the perfect tourism attraction, an easy boat ride from the Mexican tourist hubs of Cancun, Isla Mujeres, and Isla Holbox. The industry born of the phenomenon has exploded, with visitors paying $150 or more per person for guided trips to the site, where they can jump in and snorkel alongside the creatures.
The practice pumps tourism dollars into Mexico's economy and gives people a chance to see the largest fish in the ocean in its natural environment. But experts say it might also be threatening the welfare of the whale sharks, which are considered a "vulnerable" species by the International Union for Conservation of Nature.
Scientists say the whale shark population in the Caribbean, which numbers about 1,500, is generally healthy. In 2011, the biggest congregation known to science - more than 400 animals - occurred off the Yucatan.
The gathering was smaller this year. Scientists say that could be due to El Niño, increased amounts of a type of seaweed called sargassum, or a decrease in fish eggs because, for the first time, the Mexican government allowed Cuban boats to fish for bonito in Mexican waters.
Nobody knows for sure how the tourists are affecting the population.
In Mexico, the Direccion General de Vida Silvestre regulates the industry that's sprung up around whale shark tourism. At first, the agency issued just a handful of licenses to boat operators. But the number of licenses has increased rapidly, reaching 250 in 2013, 280 in 2014 and more than 320 today.
"Every year we ask them not to increase the number, and every year they increase the number," said Alistair Dove, 41, a marine biologist and director of research and conservation at the Georgia Aquarium in Atlanta.
Dove said he has counted up to 116 boats at a time scrambling for position around the whale sharks. "No one has been injured by a boat yet, but I think it's a matter of time," he said. "With that combination of inexperienced people and the sheer number of boats, it's inevitable."
It's dangerous for the animals, too. Roughly 20 percent to 30 percent of the whale sharks have been injured by a boat propeller, although regulations require that they stay 5 meters (about 16½ feet) away from the animals. There are other regulations, too. No more than two snorkelers per boat may be in the water, with a guide, at one time, and they can spend only a limited amount of time in the water. Humans are not allowed within a meter of a whale shark.
Not everyone, though, sticks to the rules. "When there's no enforcement presence out there, which there rarely is," Dove said, "it's a free-for-all."
Further exacerbating the problem, some outfitters offer guarantees that customers will see a whale shark up close. If they don't, the customer doesn't pay. As a result, boat captains do everything they can to make that happen.
"They get out there and dump everyone in the water at once to meet that guarantee," said Dove, who thinks such promises should be prohibited.
Profepa, the equivalent of the U.S. Environmental Protection Agency, is charged with enforcing the rules. La Comisión Nacional de Áreas Naturales Protegidas, which regulates national parks in Mexico, sometimes patrols the site, too, although it's not technically a park.
"It really is a bit of a Wild West operation," Dove said. "What you won't see out there with any regularity is enforcement."
David Oliver, who works with Solo Buceo, a licensed tour-boat operator in Cancun, said his company's guides have worked hard not to bother the whale sharks. He's worried, though, about all the boats at the site - some properly licensed, some not.
Dove and others are tagging the animals for tracking purposes so they can see how the presence or absence of tourists affects the animals' behavior. Anecdotally, though, he believes the sharks eat less and take in fewer calories because of the boats and snorkelers.
Ultimately, they might search elsewhere for food.
"They spend less time at the surface with their mouths open actively feeding," he said, "if they're swarmed by people."
Other countries have instituted plans to manage whale shark tourism.
In Western Australia, the government limits the number of people who can snorkel with whale sharks. A five-year study by the Australian Institute of Marine Science showed that this limited ecotourism resulted in no negative, long-term effect on the whale shark population at the Ningaloo Reef and that sharks that frequently encountered humans were just as likely to return to the reef as those that didn't.
But the limits drive up prices. Tourists there pay two or three times as much - the equivalent of $350 to $450 - for the experience of snorkeling with whale sharks.
From the beach on Isla Mujeres where our boat picks us up, it takes about an hour to reach the site. Even from afar, we see whale sharks, mouths as big as inner tubes, breaking the surface.
Our guide gives us a few tips about how to best view them. The sharks swim faster than humans, so it's no use trying to chase one. Instead, he suggests, try to intercept one swimming toward you. You'll get a drive-by you won't forget.
It's not at all scary, unless superdeep water or school-bus-size fish make you nervous. These aren't great whites - they're more like huge catfish, minus the whiskers, with neon-blue dapples covering their skin. We hang in the water, listening for the whistle of our boat captain, who alerts us when one is headed our way.
"Incoming!" I holler as one approaches. Then I take a deep breath and duck under the surface for a fish-eye view.
And what a view. The whale shark ignores me, gliding past like a silent, underwater freight train. I look in its Ping-Pong-ball-size eye, admire the rows of dots on its skin, marvel at its huge, sweeping tail.
And then it's gone.