When Linda Nelson and George Sanders purchased the warm-spring ponds west of Grantsville a decade ago, the property was a mess: a mucky party spot strewn with refuse and sprinkled with glass from broken bottles.

"It was just a big garbage dump," a magnet for wild times, Sanders says. One rumor hinted that an alligator had been set loose in the vicinity, having grown too big to remain a pet.Yet the partners, who also operate Salt Lake's Neptune Divers, saw potential there. They set about turning 80 acres just south of the Great Salt Lake into a haven for scuba divers and snorkelers. They dubbed their project Bonneville Seabase. The name reflects the springs' ancient predecessor, Lake Bonneville, and qualities the pools share with certain seas: warm, salty water and, thanks to Sanders, Nelson and friends, tropical creatures - damselfish, sailfin tangs, French angelfish, batfish and shrimp.

"This is Mitzi," said Nelson, introducing visitors to a porcupine puffer nuzzling the fingers of Trish Barrett. Barrett, a junior high school counselor and scuba instructor, dangled lettuce at the edge of a Seabase "bay" sheltered by a plastic dome.

Mitzi, a plump fish with leopard-spotted skin and thorn-shaped spikes, seemed friendly and curious but not particularly interested in nibbling Barrett's Romaine lettuce. What Mitzi, a native of the Sea of Cortez, relishes is squid.

"She's into welfare," Nelson said matter of factly. She brought out a container filled with squid portions. Barrett made an offering, and Mitzi smacked at it eagerly, making a popping-slurping sound with her fishy lips.

The free food attracted the pond's sharks.

Yes, the Bonneville Seabase has sharks - nurse sharks, a few feet long, Nelson said. The bay is home to four.

Kids who snorkel love the sharks, she said. "They're more attuned to the world" and have learned that most sharks pose no threat to humans (though they and other fish will "protect" their territory). Adult humans are more skittish.

Nurse sharks have small teeth, but they are really quite mellow.

"They're sort of the Barney of sharks," Nelson said. "They're sharks, but they're embarrassed about it. These sharks have been good ambassadors for their species."

The tropical creatures came to this near-desert inland valley in a variety of ways. Some were kept in giant tanks in Las Vegas. A few suffered illnesses there and were transported to Grantsville in hopes that they would get better. Many did, in miraculous order. Most of the fish, however, were purchased by average folks for aquariums.

"People buy them and put them in fish tanks, which is really silly," Nelson said, "because all they do is eat and grow." Some fish get too big for living-room tanks.

At Bonneville Seabase, swimmers and divers mingle with fish otherwise found thousands of miles away. People drive to Grantsville not only from the Wasatch Front but also from Colorado, Montana and Wyoming - generally in the dead of winter - to dive and to complete scuba certification training in pools ranging up to 60 feet deep, Sanders said.

"To develop an inland ocean is an incredible concept," said Scott Taylor, who operates Adventure Is . . ., a scuba-diving school in Laramie, Wyo. He accompanied a group, many of them high-school students, to Grantsville for a weekend outing in the unusual springs.

"This was just mud out here" in the late '80s, Sanders said. To firm up the spongy ground around the springs, he added rock, sprayed the mixture with salty water, which acts like a natural cement, put a construction fabric over that and finished up with gravel. The pools, too, needed dredging - cranes and buckets sit nearby - and were lined with rocks to keep the sides from sloughing off. He's still working on the site, particularly on Devil's Hole, a planned deep-diving area.

"It's a fun project," Sanders said. "We can give divers an ocean experience in Utah."

Yvette Widman and Richard Volin of Laramie were ready to give it a try. They had donned the full scuba array and were preparing to jump into an outdoor pond.

Widman, an archaeologist, said she wants to be certified to do underwater research. Ancient artifacts are found along seashores and even under modern reservoirs. "They're all over the place, like Lake Powell," she said.

Clinton Hurst, a teen swimming with friends from Lindon who had enjoyed a morning of snorkeling, was enthused about the experience. He'd never tried this in an ocean, yet the Seabase gave him a day to remember - especially meeting the sharks and feeding the fish. "You get that lettuce and wave it in front of the fish and they come to eat it," he said.

The minerals and salts in seawater are here, too, left behind when Lake Bonneville receded millenniums ago, though there is a little less sulfate and magnesium, Sanders and Nelson said. The surface temperature of the water under the Seabase Aqua Dome, posted daily on a chalkboard in the tropical-themed headquarters, was recorded on this day at 78.1 degrees Fahrenheit. The outside bay was reported to be 73.7 degrees.

The humidity under the dome is high, and the conditions stay temperate year round, Nelson said.

"This is a great greenhouse all winter long. We grow tomatoes in here - though not enough for the public," she said.

Contrary to usual expectations, the deeper you dive into these pools, the warmer it gets, Sanders said. The water bubbling up from the springs is about 90 degrees. The moisture may originate with the snow and rain that falls on the nearby Stansbury Mountains; this descends deep into the earth, where it is heated before percolating back to the surface.

Or the water may be older still.

University researchers tested the springs and found no evidence of chlorofluorocarbons - a sign of the industrial age and of modern man - in the water, Nelson said. The cycle back to the surface is therefore believed to be at least a half-century.

"My personal belief," said Sanders, "is that it is fossil water."