Underwater, everything's transformed. Sound travels four times faster than through the air, and it's hard to tell where it comes from. At first, my own breathing is all I can hear. Gradually, other sounds begin to penetrate. Soft, scraping noises turn out to be parrotfish nibbling on algae. Size, too, is distorted, magnified. The bronze barrel sponges on the seafloor look enormous, each as big as an armchair. Most amazing are the colors. Beneath the surface, pinks are blue. Reds look green. A golden sea star turns out to be bright orange when it's brought to the surface.

Reefs are the oldest ecosystems in the tropics, home to more species of plants and animals than any other habitat. Beautiful and mysterious, they also are among the most threatened on earth. The Coral Reef Alliance reports that 58 percent of the world's reefs are at risk because of development, pollution, and destructive fishing practices. More than 35 million acres of coral are already dead or damaged beyond recovery. I've come to remote San Salvador in the Bahamas to see one of the healthiest examples left. But to do this I have to learn one thing-how to dive down to the bottom of the ocean.

It's not as hard as it sounds. A boom in ecotourism has prompted many tropical resorts to offer on-site diver-certification programs, instant passports to the coral kingdom. Its vanishing beauty would be enough to lure even non-divers, but better still, San Salvador has a large variety of dive sites. You can explore reefs, shipwrecks, blue holes, even vertical walls -- the massive underwater cliffs that can slope down hundreds of feet into the ocean depths. With only two operations on the island, Club Med and Riding Rock Resort, no more than 1,500 people dive these reefs a year. What's more, while resorts now routinely teach divers how to explore reefs without damaging them, Club Med also has installed a mooring system that eliminates the need to drop anchor -- a practice that can rip the delicate structures.

BREATHING LESSONS
On the boat dock of Columbus Isle, the 286-room Club Med village, my course instructor, Cale Sterloff, explains the week's itinerary. We'll study and drill at the resort's dive center -- one of only three Club Med classifies as "dedicated," which means it offers not only classes but the staff and equipment to take guests on as many as three dives per day at more than 40 sites.

After four days, I should be a PADI-certified, open-water diver, able to monitor my air tank, check my depth, and use a compass to find my way back to the boat. PADI, or the Professional Association of Diving Instructors, is the world's largest diver-training group. Members, just by showing their PADI cards, can rent equipment or hire dive guides in almost any country in the world.

On the dock, we practice screwing our steel regulators onto tanks of air, then fumble with our hoses, each attached to something different -- a mouthpiece, a gauge, an orange vest that controls our buoyancy. With the grace of a lumbering sea lion, I follow Cale and step off the dock ladder. As soon as our group is ready we push the vests' deflate buttons and slip down through the water. At about 12 feet, we reach the sandy bottom, gathering around Cale as he pantomimes basic skills. We mimic him -- blowing bubbles, flooding our masks, then using our noses to clear them of water. It feels natural sitting on the ocean floor, but maybe that's because I can see the dock pilings a few feet away.

Two days later we board the resort's dive boat, the Eagle Ray, for our first open-water dive. While I nervously prepare my equipment and put on my wet suit, dive master Adam Belgrade attaches the mooring and describes the site. One by one we jump off the boat and, led by Cale, drift down slowly, gliding over the top of a reef. Sea grass flutters like ostrich feathers in the current. A school of blue tangs moves past in a shimmering curtain. The reef bustles with so much activity that the surrounding ocean and sand seem deserted.

With no plankton to cloud the water, the visibility is near perfect. Divers in San Salvador tell of being 60 feet down on a night dive and lying on their backs, counting the stars. "It's amazing here," says one of the more experienced divers. "I've never been anywhere you can look down 90 feet and read the brand name on a diver's fins." He's interrupted by a whale sighting. As the boat slows, we watch five pilot whales cavort in the waves. They come up for air, showing off curved black backs the size of Volkswagens.

These waters teem with animals. "Once in a while you see a movement out of the corner of your eye and realize it's a hammerhead shark," says Jean Paul Morales, who heads Club Med's dive center. "We counted a school of 37 hammerheads in April." The thought of swimming with sharks makes me want to turn in my flippers, but Cale and the other divers reassure me. "They're harmless," they say. "Sharks don't attack unless provoked."

On my next dive I feel more sure of myself, descending on my belly so I can see where I'm going, and scanning the depths. My heart stops as I catch a quick movement. Instead of a hammerhead shark, I'm rewarded with the sight of two sea turtles. They practically soar as they swim away, their graceful motion nothing like their slow crawl on land.

ROCK OF AGES
The more you dive, the more you understand that everything revolves around the massive coral reefs. Coral polyps are actually animals, living on top of rocky structures made from their dead ancestors. Over tens of thousands of years these have grown into the mountain-like reefs that provide homes and food for most ocean life. Underwater, I look out for staghorn and elkhorn coral, the giants that can rise straight up out of the sand. There are coral colonies made up of velvety-green lettuce coral, sandy-beige brain coral, and golden cactus coral. These are the hard corals, the reef builders, whose skeletons will form yet another layer of limestone. The flashy corals -- red gorgonia, bright orange tube coral, lavender sea fans -- are soft, flexible enough to flutter gracefully at the slightest current but unable to build reefs.

Reefs depend on a symbiotic relationship between plants and animals, between algae and coral. Coral gives off wastes that are used by the algae. The algae, in turn, manufacture food for the coral. Algae also live inside the coral, making the exchange easier. "It's a delicate balance," says Olin Feuerbacher, a marine biologist at the University of Arizona. "Under stress, coral will expel its algae, which can lead to the death of a reef." Stress comes from almost anything: pollutants or a kick of a diver's fins that scrapes off a layer of the coral's protective mucus. The water around a dead reef is cloudy, dense with algae blooms. The hard corals become bleached as the algae filters out sunlight. Even here, I see patches of white where the algae has been expelled and all that's left is a pile of skeletons.

On healthy reefs, every nook seems to house an animal. A spiny lobster makes its home in a small crevice, and arrow crabs strut by on spidery legs. Parrotfish, known as the cattle of the sea, graze on the coral itself. They're nibbling on samples of geologic time, used by scientists and archaeologists to learn about how and when the earth was formed. "These living reefs are natural laboratories," says Feuerbacher. "They re-create the conditions that produced ancient rock." Twenty feet down, this reef dates from the time of Columbus; 85 feet down marks the time of Jesus.

NEON DARKNESS
After four days of classes and drills, I'm certified. To celebrate, a classmate and I sign up for an all-day excursion to a section of reef riddled with holes and caves to explore. Down 60 feet, we follow our dive master toward a dark, chimney-like hole in the reef. We deflate our vests and enter. As my eyes adjust, I can't believe the colors. Tiny yellow sea sponges glitter like gold coins. Red coral sparkles against walls of velvety green. I spiral down to a funnel-shaped bottom that opens out into an endless stretch of electric blue, what divers call the Big Blue. It's as if I'm skydiving in slow motion.

Reluctantly, I head toward the top of the reef. With 40 feet to go, I pause for a last look. Below me, yellow coral trees wave their branches over a patch of sea whips and fans. Finally, I head up. As I surface, I break into a huge grin, giving the okay signal to the captain.

Later, when I close my eyes, I still see the swaying aquatic garden. Never again will I look at a piece of coral and think it's just rock. I've seen the reef, and it's alive.