It looked a bit like an Aboriginal painting. The great blue canvas of the Pacific was stained with pale brown tide lines, circles, whorls and blotches. Surely not pollution, I told myself, here of all places. I was flying from Dunk Island in the southern part of the Great Barrier Reef to Cairns on the mainland.

When, six days before, I had flown from Cairns to Lizard Island, at the northern end of the 1,250-mile reef, there had been no trace of these strange brown manifestations. Instead, the uniform azure of the sea was broken only by the colours of the coral cays: turquoise, tourmaline, amethyst and lapis lazuli. Most were submerged, others showed just enough coral above the water to support a golden strip of beach and a few palm trees.

Then I remembered what Kym, the head boatman at Lizard, had told me: every year, around mid- November, four or five days after the full moon, the coral spawns — the polyps, tiny little creatures that build the limestone mosaic of the reef, erupt in an orgy of procreation, their eggs turning the water into a thick soup. “Sometimes it looks like a smoky haze rising from the water,” he added. So, evidence of coral spawning is to be welcomed — in sharp contrast to coral bleaching.

Caused by global warming, bleaching is the biggest threat now facing the world’s coral reefs. The Maldives are one of the worst affected, with up to 95% of their coral dead. Australia’s Great Barrier Reef, although it may look in good shape, “is right in line for major damage”, according to Dr Charlie Veron, an eminent Australian marine scientist.

In the past, bleaching occurred only periodically — every 40 years or so — giving the coral a chance to recover. Now that it is becoming increasingly frequent, the prospect is grim. For the time being, however, the place is a paradise.

Kym had taken me on a tour around Lizard, including Seabird Island, a small rocky cay rising only a few feet above the Pacific. It was an amazing sight, covered with hundreds of arctic and little terns and white-capped noddies with their chocolate-brown bodies. The terns, which had probably just arrived from the Arctic Circle, were lined up on the flat rocks as if on parade, beaks to the wind, while the noddies flew about restlessly, twisting and turning in the air like a corps de ballet and screaming as loudly as any diva. A small group of ruddy tumstones, small waders, were searching for crustaceans at the water’s edge. “They’ve just come from Spitsbergen to winter here,” said Kym. “It must be a 10,000-mile flight — incredible how such small birds can do it.”

Lizard is about halfway between the Australian mainland and the outer reef, where you can spend the day diving or snorkelling. Unfortunately, once we left Lizard, the Pacific began to flex its muscles. Soon, the big dive boat was pitching and rolling. “’Fraid we won’t be able to make the Cod Hole,” said the skipper, steering with one hand. “What’s the Cod Hole?” I asked. “The best place for diving on the reef. When it’s calm you can feed the big potato cod by hand. Provided you’re wearing a chain-mail glove.”

“There it is, right ahead,” he said a minute later. “The only gap in the reef. Today, the whole of the Pacific Ocean is trying to force itself through that gap.” In 1770, a desperate Captain Cook spotted the break from the top of Lizard Island and, after months of frustration, finally escaped from these treacherous waters.

We turned back to calmer waters, anchoring half a mile away, above No Name Reef. Apart from the divers, we were four snorkellers, including an Italian couple, both of whom wore wet suits, and in the lady’s case, shocking-pink flippers. Under the command of Christian, a blond giant from Germany, we glided through water clearer than any I had swum in before, the colours — blues, greens, browns, purples and black — more vibrant. Feeling rather like a whale among minnows, I found myself in the middle of schools of small green damselfish and a host of mainly yellow and blue angelfish, bannerfish and butterfly fish.

The human eye, I discovered, is very different from that of a fish. Swimming right up to me came a tiny fish of a vivid electric blue. It turns out, however, that although flashing neon to the human eye, it is virtually invisible to predators. I eyed a buxom coral trout with blue spots on a pink background — they make good eating — and listened to the parrotfish gnawing the coral.

We were now traversing a great garden of corals, of which the Barrier Reef has 350 different hard varieties and about 60 soft, many of them exotically coloured. I passed over a huge boulder-sized brain coral then came to what seemed at first sight a petrified forest in bright blue — staghorn coral. I was admiring plate coral as big as a dining-room table when I spotted the crinkly blue lips of a giant clam. Although motionless, it looked as if it might snap shut if provoked.

Christian dived again to point out a puffer fish on the bottom, bloated and fearsome-looking.

I swam to the edge of what looked like a chasm. The water shaded from palest to darkest blue and finally black. I felt a thrill of excitement as I floated weightless, suspended over the void. Hundreds of small fish twinkled in the refracted sunlight. Some came so close I could look into their unblinking eyes, and see their gills working. A few minutes later, Christian bobbed up to tell us we were drifting too far from the boat. As I made to follow him, a pair of shocking-pink flippers cut across my bows, but even that could not spoil my enjoyment.

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