After tying up to the mooring at Ribbon Reef 10, or Cod Hole, we geared up and hit the water for the first dive of our trip. We followed the mooring line down to 80 feet and swam northeast through a series of long, narrow coral ridges affectionately called “Supermarket Aisles.” A curious 5-foot Napoleon wrasse glided by and shadowed us for most of the dive.

It was fun searching for pigmy seahorses on the gorgonian corals and checking out the clownfish darting in and out of the giant anemones. Below the coral overhangs, sweetlips and resting green turtles swayed back and forth in the gentle current.

We ascended gradually along one of the aisles until it opened into the flat, sandy lagoon at 40 feet called Cod Hole. The shallow site was a favorite stopover for fishermen plying the outer reef. Rumor has it that the fishermen often used the lagoon’s calm water to clean their catch. As a result, the resident grouper, or potato cod, grew fat on the fish scraps that regularly floated down to them.

Eventually, these big fish started to attract divers, who in turn started hand-feeding them. When the fish grew tame, the Australian government designated the Cormorant Pass Section of the reef a protected area and established rules on feeding the cod. Today, the site is part of the Cairns Section of the Great Barrier Reef Marine Park.

Within seconds of reaching the Hole, a dozen 6-foot potato cod (Queensland grouper) surrounded us. A 300-pounder approached me calmly and nudged my side to see if I had anything to eat in the outside pocket of my buoyancy compensator device. Finding nothing, it moved over and casually bumped another diver.

When our dive guide took out his little plastic box of fish scraps, however, things quickly changed. Eager cod were everywhere, darting in and around us, jostling for position, their gaping mouths vacuuming up everything that resembled a scrap of fish. It was amazing how fast these big fish could move, sort of like excited Labrador dogs. During the fray, an errant cod tail dislodged one diver’s mask and knocked the regulator out of another’s mouth. The feeding frenzy only lasted a few minutes; and when our guide showed the cod the empty container and gestured that it was all gone, the fish immediately returned to their relaxed demeanor and lumbered around us, posing for pictures.

The Great Barrier Reef, the world’s largest reef system, boasts 2,900 individual reefs and 900 islands stretching more than 1,600 miles through the Coral Sea along Australia’s east coast. It is also the world’s largest single structure made by living organisms and is easily visible from space. UNESCO designated it a World Heritage site in 1981, and CNN recently listed it as one of the Seven Wonders of the Natural World.

The 65-mile-wide reef has countless dive sites that contain more than 400 species of coral and 1,500 species of tropical fish. It’s also home to six varieties of marine turtles and 30 species of whales, dolphins and dugongs (manatees).

The GBR broke onto the world stage in 1770 when the hull of Captain Cook’s Endeavour abruptly slammed into the kingdom of the coral polyp. After patching the gaping hole in the ship’s hull, Cook spent the next three weeks sailing south trying to find a safe passage out through the reef.

Today, a flotilla of modern dive and snorkel boats ferries adventurers to the reef from Australia’s northeastern ports. Here the reef hugs the shoreline (from 10 to 60 miles off shore), allowing for easy day-trips from gateway ports like Cairns, Port Douglas and Cooktown. Snorkelers usually explore the reef on day-trips, while divers typically spend four to nine days on liveaboard dive boats.

On our seven-night Coral Sea safari, we visited about 30 dive sites. After our potato cod encounter, we motored well beyond the main reef to Osprey Reef in the Coral Sea. Situated about 200 miles north of Cairns, this isolated, 30-square-mile, oval-shaped atoll boasts some of the best oceanic pelagic diving in the world. Pelagic is the collective term for big fish such as manta rays, sharks, marlin and tuna.

Diver-sized soft corals and huge, golden gorgonian fans blanket sheer undersea walls that plummet 3,000 feet on the outer side of the atoll. This is the domain of hammerhead, whitetip and gray reef sharks attracted by the large schools of trevally, mackerel and tuna. In sharp contrast, clouds of small reef fish float above the kaleidoscope of colorful coral gardens that crowd the shallow lagoon inside the reef. Manta and eagle rays, green turtles and schools of barracuda patrol the deep cuts through the fringing reef.

After sailing all night, we arrived at Osprey Reef early in the morning and dropped anchor in the calm waters of the 100-foot-deep lagoon. We started our two-day Osprey Reef adventure with an orientation dive at False Entrance, a deep gully on the western side of the lagoon. The large school of barracuda that blocked the gully entrance slowly parted as we swam through. A few fin kicks beyond the barracuda, a giant manta filtered plankton from the mild current running through the gully. Below it an 8-foot whitetip shark rested on the sandy bottom. Not a bad way to start a day of diving.

The highlight of our trip took place on the last day on Osprey Reef at a popular shark site called North Horn on the northern edge of the atoll. The site features a large coral amphitheater where the Pacific current bends around the reef. We formed a tight semi-circle on the bottom, our backs to the coral wall. A dozen or so silvertip, whitetip and gray sharks circled slowly over our heads as we settled in.

Once in position, a crew member on the boat above lowered a sealed garbage can of fish heads and scraps to the reef about 20 feet from us. Within seconds, 40 to 50 anxious sharks started circling and bumping the bin, trying to dislodge fish scraps through the small holes punched in the can’s side. Then our dive guide pulled a rope that released the can’s cover, and all heck broke loose.

Fish heads were flying everywhere as shark after shark tore into the scraps. The scrap fish ball quickly morphed into a shark ball of fins and slashing teeth. You could actually hear the sharks crunching down on the heads. A couple of large moray eels, some big jacks and a huge potato cod also joined the frenzied feast.

It was one of the most electrifying dives in my 25 years of diving, a high-voltage experience with big sharks and jacks passing within touching distance. It took about five minutes for the last fish scrap to disappear into a shark’s mouth, and the crew hoisted the banged-up bin up to the boat. The sharks followed the can part of the way up, but dropped back and resumed a more relaxed behavior, slowly swimming around us — perhaps hoping for an encore.

Two days later, we were back on the main reef exploring more sedate sites, the memories of the sharks still lingering in our minds.

The Great Barrier Reef offers spectacular year-round diving experiences, but some months command more attention. Hundreds of dwarf minke whales migrate to these waters from May to July to mate and bear young. Making close eye contact underwater with one of these great mammals and feeling their vocalizations reverberating through your chest is as good as it gets. October to December is coral spawning time. Spawning occurs at night when the coral polyps release clouds of sperm and eggs into the water column. All this food in the water attracts thick schools of baitfish, which in turn draw in the big pelagic fish. February to May (Australia’s fall) offers cooler temperatures, excellent visibility and lots of reef fish activity.