I'm hanging in a bottomless abyss of blue-green, peering through a mask, when a shadowy shape practically the size of a semitrailer truck glides into frame.
A 40-foot fish swims past, its mouth open so wide I can see down its throat as it scoops up millions of tiny, nearly invisible fish eggs.
I'm awe-struck. As a scuba diver with more than 300 dives under my belt, I've long dreamed of catching a glimpse of a single whale shark. Today, I'm surrounded by dozens of the giant plankton eaters.
About a decade ago, Mexican fishermen discovered that whale sharks were congregating in the warm waters off the Yucatán coast. The sharks, which they called "domino" or "checkerboard" fish, gather here during the summer months for food — they gulp the clear, pinhead-sized eggs of spawning little tunny, a type of tuna.
Big, graceful and toothless, the sharks soon became the perfect tourism attraction. In the past six or seven years, the industry surrounding the phenomenon has exploded, with visitors paying $150 or more for guided trips to the site, where they can jump in and snorkel alongside the animals.
"After a while tour operators started to see that it was a great experience," says David Oliver of Solo Buceo, a dive shop in nearby Cancun that we hired to ferry us to the site.
The trips are popular because, unlike scuba diving, which requires certification, even kids or grandparents can snorkel.
Oliver still remembers the first time he saw a whale shark up close. "The water conditions were not too good; it was green with no visibility at all," he said. "When I saw it, it was already a couple of feet away from me, with the big open mouth."
Who wouldn't want that same experience?
Today, tour operators are required to follow a strict set of regulations — dropping no more than two snorkelers in the water at one time and limiting the amount of time each spends in the water — but that doesn't always happen. Scuba diving isn't permitted; snorkelers aren't allowed to touch the sharks, and they must stay at least a meter away from the animals. Only biodegradable sunscreen is allowed, because other types could harm the plankton the sharks eat.
An unforgettable drive-by
We've timed our trip to coincide with the peak of whale shark season, mid-July, but the sharks can usually be spotted here from mid-May to early September. From the beach on Isla Mujeres where our boat picks us up, it takes about an hour to reach the site.
As we approach, we slow to a crawl. We can see the whale sharks from afar, their big black fins slicing the surface, mouths nearly as big as inner tubes breaking the surface.
Our guide gives us a quick briefing, then 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 that's swimming toward you. With a little luck — and we have plenty — you'll get a drive-by you won't forget.
It's not at all scary, unless superdeep water or school bus-sized fish make you nervous. These aren't great whites — they're more like huge catfish, minus the whiskers, with gorgeous neon blue dapples covering their skin. We hang in the water, listening for the whistle of our captain, who alerts us when one is headed our way.
"Incoming!" I holler. 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 pingpong-ball-sized eye, admire the rows of dots on its skin and marvel at its huge, sweeping tail.
And then it's gone.
We've brought our own masks, fins and wet suits, but most companies provide gear if you don't own your own. Our operator has also brought fruit, drinks and a stash of baguette sandwiches for when the post-snorkeling munchies hit. That's what we're doing now — eating lunch, admiring whale sharks from our boat.
"I wanted to swim with the whale sharks because I love seeing how animals live and move and feed in the wild," said my sister Angela Pierce, a pediatric brain cancer researcher at the University of Colorado-Denver. "Because they're so beautiful with their spotted skin. And because I like thinking about all the different fish and animals and invertebrates and plants in the sea and how their life cycles mesh together."
It's mind-boggling, really. How can such a large creature get enough substance from food so tiny it's barely visible? And what do they think of us, mesmerized by their every move? They seem baffled by our interest.
I could stay here for days, I think, pondering the mysteries of whale sharks. Instead I'll have to make do with a few days, and the hope of a return trip someday — if the sharks stick around that long.