The dive master said the sea floor was 3,000 feet straight down. "You don't come back from that depth," he said, laughing blackly.

With that thought, I stepped off the boat. The bright blue sky disappeared. My eyes opened on the deep blue of the Coral Sea.

The water was nearly clear as glass. Twenty feet below my flippers, two 10-foot gray reef sharks and more than a dozen smaller black- and white-tip reef sharks patrolled in smooth ellipses.

My dive partner Kyoko Kishi hung weightless in the water 10 feet below, waiting. Her exhalation rose in a silvery cascade toward a reunion with the surface.

When I reached her, we exchanged OK signs and followed the other divers down, past the sharks, to a horse-shoe-shaped shelf on Osprey Reef, where we sat down like sports fans in a stadium. Soon 30 divers had perched there on the lip of an extinct volcano. Sharks gathered overhead, vehicles of muscle and cartilage that glided as effortlessly as eagles in the sky.

Nothing in 400 million years of finely honed instincts tells sharks that an awkwardly moving creature attached to a metal can and blowing bubbles is edible. But they're not stupid. They were circling because they knew food was coming.

Soon a crewmate from our dive boat, the Taka, would dump a half dozen 10-pound tuna heads chained to a cable over the side of a raft, which we could see floating overhead. We would then witness the drama of a feeding frenzy on one of the world's great natural stages -- the Great Barrier Reef.

The reef is a most unusual wilderness, and not just because it's underwater, or that it's the largest living structure on Earth, or that it's populated by a parade of improbable animals. It's because it's so robust. At a time when many coral reefs have died off or are in danger of it, the Great Barrier Reef thrives. It is one of the world's last best places: an oceanic ecosystem in its glory.

As pristine places like the Great Barrier Reef become more scarce, they also become more desirable. Nearly 2 million people visit the reef each year, making it one of Australia's most popular attractions. Tourism here is a powerful economic force. Marine tourism alone generates more than $1.3 billion Australian dollars annually, and land-based tourism on the coast near the reef brings in nearly $3 billion more. Travelers are drawn to the spectacles a healthy reef affords. Such as a sizable population of big predators.

With a barely visible splash, the tuna heads hit the water, and the sharks went into a frenzied scrum. The biggest ones, who were first with a jaw grip, tugged and twisted at the chained-up tuna heads. Others charged the knotted crowd, vying for an angle on the meat.

When cruising, sharks look menacing, but stately. With hardly a visible flick of their tail fins, they swim with liquid grace. Now, they bolted past at lightning speed. Reliable estimates suggest some sharks can hit 40 miles per hour; as one flew by with a hunk of tuna head in his mouth, knocking over one of the seated divers with the force of his wake, that seemed entirely plausible.

In five minutes, the thrashing was over. The cable hung empty and slack. The sharks dispersed.

An underwater tour bus

Kishi and I slowly made our way back to the boat, surfaced and spit out our regulators, exchanging the dry, sterile air of the tanks for the humid, oxygen-rich atmosphere of the tropics. We stripped out of our tanks, buoyancy-control vests, weight-belts, masks and wet suits. Still, having dropped 45 pounds of gear, as I headed into the galley for lunch, I felt the terrible burden of gravity. For the past 35 minutes underwater, I'd been weightless.

The Taka is one of the biggest of the dozens of live-aboard dive boats that ply the Coral Sea. It accommodates as many as 30 passengers, along with a crew of 10. Divers from Egypt, Japan, Poland, Spain, Finland, the Netherlands, England, Australia, New Zealand, Canada and the United States filled every available bunk and suite.

The usual quirks of group travel applied. The Finnish woman dove in her pajamas instead of a wet suit, and she generally ignored her assigned dive buddy, the Egyptian doctor, who was exasperated by her unpredictability. The Polish woman was a rock star going by the name Dominika; she showed us one of her videos on the big-screen TV in the galley. Sporting teased-up blond hair and strutting in a skin-tight pinstripe suit and stiletto heels, she snarled, "I'm going to unleash myself, release myself on the world." There was something about her feral expression that made the shark feed seem almost genteel.

Life in every niche

Starting in Cairns, the most popular tourist port on Queensland's coast, the Taka traveled 200 miles to Osprey Reef and back, stopping at the prettiest, most pristine dive sites available en route. Nothing that happened in my four days on the boat compared with seeing the reefs themselves -- conglomerations of life that can be seen from outer space. I often found myself hovering in one spot, admiring the way the reef thrums with creation: Seussian Christmas tree worms swaying in the current, a neon-painted slug called a nudibranch scooting across a plate coral, translucent cleaner shrimp picking the nits off a gasping moray eel in his chosen crevice. Every square inch is occupied by somebody trying to make a living.