It might have been the setting for a "Jaws" movie.
Six snorkelers wading like ducks in a row, cruising just below the surface of the water while watching exotic fish dart beneath them. It was all very peaceful, until the mysterious whale shark appeared out of the deep blue.
The whale shark is one of the most perplexing and elusive creatures in the ocean, still largely a mystery even to the marine biologists who have dedicated careers to studying the creatures.
But here, in the confines of the Georgia Aquarium in downtown Atlanta, it's impossible not to see the giant whale sharks — particularly when you're in the middle of their fish tank.
It's also somewhat hard to avoid them: The creatures seemed more intrigued by the visitors, often lumbering toward them like a slow, curious locomotive.
The guests were circling the world's largest fish tank through the aquarium's "Swim with Gentle Giants" program, which plucks six snorkelers and six divers into the 6.3-million gallon fish tank each day.
The visitors are treated to close-up encounters of roving bands of sting rays, sleek hammerhead sharks, enormous grouper and countless other species. But the puzzling whale sharks were the real draw — and for good reason.
The aquarium is the only outside Asia to house the whale sharks, and the only in the world to offer tourists a chance to dive with the creatures. The program's directors pitch it as an innovative and safe way to help visitors better understand animals they'd otherwise never see.
"An immersion experience is the ultimate way of connecting people and animals," said Bruce Carlson, the aquarium's chief science officer.
"It's a real opportunity for us to expand ways for people to get to know the animals here at the aquarium and a once-in-a-lifetime opportunity for our visitors to see animals they'll probably never get a chance to see in the wild."
But the ambitious program has raised concerns from critics who worry that dangling legs and curious tourists could stress the habitat of the whale sharks and thousands of other animals that share the massive tank.
"There's a chance these animals can become stressed because of the increase in the amount of people in their environments," said Lori Marino, an Emory University biologist who studies whale biology. "Not only can it affect their physical health, but their mental health. And we don't know how much stress this puts on the animals or how they could respond."
The Georgia Aquarium is one of the few places that have ever attempted to house the creatures, and the only in the U.S.
So far their record is spotty: Two of the whale sharks have died since the aquarium opened in 2005. But the aquarium has invested in research projects on the whale shark in Mexico, Taiwan and Mexico. And the facility is quickly making a name for itself in the research community for its whale shark work, thanks to divers who have already logged thousands of hours feeding and studying the massive animals.
Carlson said he gave the go-ahead to the new program because the dives have so far had "no effect on the whale sharks' behavior."
"We're the experts on that, and we can make the judgment because we probably spend more time with whale sharks than anyone criticizing us," he said.
"Most people who have contact with them have probably had a minute-long experience in the ocean. You have to trust our judgment on that. We've gotten to understand their nature, and we feel quite confident that our presence is not affecting them."
In many ways, the aquarium is charting new waters. A handful of other facilities offer diving or snorkeling experiences in their tanks, but none offers a setting as expansive as the Georgia Aquarium's Ocean Voyager tank.
Along with the whale sharks, the salt-water exhibit is home to thousands of other animals, including the largest collections of giant grouper, wobbegong sharks and a dozen other rare species.
The dive is far from a free-for-all. During a 15-minute briefing, guides stress a message of conservation and warn participants not to touch any fish while in the tanks. They then don wet suits, snorkels, masks and flippers before plopping into the salty water.
The snorkelers are forced to stay in a rigid line during the swim, kept in toe by dive guides and staffers armed with underwater cameras to document their journey.
It's not a cheap trip, costing $190 for snorkelers and $290 for scuba divers. But the aquarium has so far been encouraged by the response. Aquarium spokesman Dave Santucci said some 1,500 have signed up for the program before its June 8 start.
Krista Massey, an aquarium member who previewed the program, said she felt unnerved when she dipped into the water.
"You know, there's hammerhead sharks in there," she said. "There's all kinds of predators you've been scared of all your life, and all the sudden, you're in their world."
It didn't take long, though, for her to feel more comfortable in the tank — and the experience gave her a deeper sense of wonder for the creatures floating around them.
"What you learn through the process is so much more than what most people know about these animals, and it teaches you a different type of appreciation," she said.
Under the placid surface, too many fish to name swarm below, exotic creatures like zebra sharks, cownose rays and guitar fish. A group of glimmering Golden trevally shimmer around one bend. Not far from there, Grumpy the Grouper, a local icon, glowered near a window.
And of course, there were the four whale sharks, massive beasts that barreled around the tank as they wished, sailing agonizingly close to the awed visitors.
As the 30-minute experience neared its end and the six lined up to depart the cage, the silence underwater was suddenly broken by a warning from a guide.
"Stay flat," she yelled. "Feet up."
Then, a current rippled through the water. And the gentle giant swam by once more.