There’s nowhere in the world quite like the Great Barrier Reef. It originated more than 18 million years ago, though most of the present reef is a mere 2 million years old. More meaningful to nongeologists is the fact that the Great Barrier Reef is more than 2,200 km long – about the distance from London to Athens, or New York to Dallas. The reef starts at Lady Elliot Island in the south (off the coast at Bundaberg) and stretches far up beyond the northern tip of Australia to Papua New Guinea. At some points it’s just a few reefs or cays poking up out of the sea, at others it’s more than 70 km wide.

The entire reef was made by a tiny primitive polyp (of the coelenterata family), a relative of the sea anemone. These apparently insignificant creatures are responsible for the largest structure made by any living creature – which  is also the largest living structure in the world. Yes, the Great Barrier Reef is alive and growing. The polyps, which are almost as primitive a life form as you can get outside a rugby scrum, form together in large interlocking colonies (on much the same principle as a scrum). Here, instead of heaving and grunting and punching one another, they excrete lime, and when they die their coagulated skeletons remain as brittle coral.

New polyps then take their place on top of the old skeletons, and over countless millennia this process gradually expands into the reef. The polyps which produce the coral require clear water and a temperate but salty sea. If the water temperature dips below 17°C the coral can’t grow, which is why the reef doesn’t extend south along the entire eastern coast of Australia. Sunlight is also necessary to the coral-making process, which is why no coral grows lower than 30 m or so below the surface.

The Great Barrier Reef runs along the edge of the Australian continental shelf, sometimes as far as 300 km offshore. The channel between the mainland shore and the reef is comparatively shallow in nautical terms, seldom more than 50 m. The coral foundations of the reef often rise sheer from this depth. But if coral can’t grow any deeper than 30 m below the surface, how did this solid foundation of the reef come into being all those millions of years ago? As usual scientists have come up with all kinds of theories, but the fact is no one knows for certain. However, a marine biologist friend of mine thinks there can only be one plausible solution. The sea was shallower at the time when the corals first started to form the Great Barrier Reef, and as the sea level rose, so the corals gradually grew at the same rate. My suggestion that the sea bed may simply have sunk was dismissed out of hand, for baffling scientific reasons which remain beyond my comprehension.

The Great Barrier Reef consists of more than 400 different types of coral, which account for its huge variety of shapes and colours. Spectacular evidence that the reef itself is still alive and growing comes every year after the full moon in early summer. This is when the corals spawn, and their countless tiny eggs scatter through the sea like a suspended submarine blizzard amidst the clear blue water.

If the Great Barrier Reef were just coral, it would still be an aquatic wonderland (and for once this is no cliched exaggeration). But what makes it even more spectacular is the presence of many thousands of different species of marine life, which have made the reef their home. It has been estimated that more than 10,000 different species inhabit the reef, including more than 2,000 varieties of fish, among them many exotic tropical species. There are dugongs (sea cows), migrating whales from the Antarctic, and of course sharks, which brings us to the dangers of the reef.

For years there used to be legends of unwary fishermen devoured by giant squid, enormous clams which swallowed people whole, octopuses capable of snatching sailors from the decks of ships, and packs of killer ‘jaws’ which swallowed entire shipwrecked crews turning the sea red. Imaginative depictions of these scenes adorned many a 19thC parlour wall. Yet curiously, all these exotic beasts suddenly vanished around the time when photography was invented.

The fact is, there are dangerous species on the reef. But they’re not monsters, and they’re few and far between. Reef sharks don’t eat people. The stingrays may look hideously menacing, but they’re shy and tend to avoid divers (but you should avoid disturbing them: that evil-looking tail really can sting). However, if you swim in waters close to the shore you may encounter box jellyfish (sometimes known as sea wasps), which, during the summer, can be a menace on the tropical beaches, and if you’re stung badly it can be lethal. Just make sure you ask a local if there are any before having a dip on a beach where no one else is swimming.

Your most likely mishaps are likely to be cuts and grazes suffered from brushing up against coral when you’re diving. Grains of coral often remain in the wound, and can cause bad inflammations. Make sure your wound is properly cleaned as soon as you get back. Also, be sure to wear something on your feet. You may not notice a stonefish disguised as a rock, but you’ll certainly notice its poisonous spines when you tread on one.

Other dangerous beasties – such as scorpionfish and cone shells – are fairly rare. If in doubt, just leave well alone. There’s no need to touch anything: and snapping off bits of coral just means there’s less of the reef for the next visitors. (Just think of the years it took to grow that exquisite little twig of coral you fancy as a souvenir. It won’t look half so good out of the water, and it’ll probably end up in the attic anyway.)

Opinions vary about the overall health of the reef. The experts tend to the opinion that most of it is in fine shape. But there’s no avoiding the fact that parts of it are definitely in poor shape. This is largely due to human interference – clumsy anchors, pollution, emptying sewage, souvenir hunting etc. Fortunately for us, there’s also an animal scapegoat: the dreaded crown of thorns starfish. Horror of horrors: these tend to appear in sudden swarms and actually devour the coral, although some scientists have suggested that they only swarm because of human-induced imbalances in the marine ecosystem. And apparently they only devour coral in the absence of their usual feeding prey (molluscs, and so on) – which is usually due to pollution.

Despite all this, the Great Barrier Reef remains one of the world’s marine marvels. If you dive or snorkel here, it’s guaranteed to be one of the highlights of your visit to Australia.

At popular Fisherman’s Beach you’ll find all the usual water sports. But you don’t have to walk far to find a comparatively deserted spot. There are a number of paths which spread out across the 6-km-long island. You can head for a distant beach (such as Butterfish Bay in the north), climb Mount Windham, or strike out for the lighthouse at Bald Rock Point (which involves a six-hour return walk). Those who haven’t come all this way for a commando endurance course may prefer to take a cruise from Fisherman’s Bay. At 10 am you can catch the three-hour round-the-island cruise and be back in time for lunch. There are also snorkelling cruises, sunset booze cruises, and so on.

The best snorkelling is half an hour’s walk across the headland south of Fisherman’s Beach at Monkey Beach, where you can also see a reminder of the island’s sad (and largely forgotten) history. The shell mounds on this beach are the only remaining traces of the indigenous Aborigine tribe, shipped out as slaves to Fraser Island by early European arrivals.

The celebrated Great Keppel Island Resort, at the eastern end of the island, was closed at time of writing for a refurbishment costing hundreds of millions of dollars. The aim is to make it sustainable and eco-friendly. There is other accommodation on the island – see Eating, sleeping and other useful information in the main section.

Middle Island is the small island just west of Great Keppel, and is a national park. It has trees, a campsite and a nearby underwater observatory – complete with a genuine Taiwanese junk. Apparently the junk was confiscated for illegal fishing and then sunk to provide a shelter for the local tropical fish. I had no alternative but to believe this story, owing to the intimidating hugeness of the red-faced ranger who told it to me. You are free to doubt such local lore at your peril. The waters around the point are excellent (or notorious) for spotting sea snakes.

The 1-km-wide island just south of Great Keppel, Humpy Island, is named for its geographical characteristics. It also has a rather dismal, windy campsite.

North Keppel is the second largest island of the group and lies 10 km or so north of Great Keppel. It has a pleasant campsite in the dunes on the north-west of the island by Considine Beach. There’s some fine swimming at several beaches, and no real development. Alas, in some parts of the island, residents outnumber visitors by more than 100 to one, though why so many sandflies should have chosen to take up residence in this spot remains a mystery (even to my unimpeachable source). However, this is not a problem over the entire island.