updated 3:23 p.m. ET June 17, 2008

ATLANTA - 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.

If you go Georgia Aquarium: 225 Baker St. NW, Atlanta; or 404-581-4000. The dives occur daily, 3 p.m. for scuba divers and 4:30 p.m. for snorkelers. Scuba divers are charged $290, snorkelers $190. Guests must be 12 years old to participate, and those between 12-18 must have a participating adult with them.

"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.

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"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.

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