From The Sunday Times Travel Magazine

Dismemberment insurance: two words you don't really want to hear when booking a holiday. But that's what I've been recommended for my diving trip to the Galapagos. Just in case. Most people have heard about the exotic menagerie that has been busy evolving in the Galapagos for the past few thousand years. Cast adrift on a raft of lonely volcanic islands are tortoises half the size of Volkswagen Beetles, sooty-black marine iguanas and comical blue-footed boobies. Then there are the deceptively drab-looking finches that fired Charles Darwin's imagination. Their uniquely adapted beaks helped him form his theory of evolution, changing perceptions of nature for ever.

The number of visitors who make the long journey out to this incredible Pacific archipelago has grown steadily in recent years. Well-equipped tour boats are a common sighting, as they disgorge orderly files of novice naturalists in khaki shorts, sensible sandals and sunblock. How many of them realise that below the blue swell is another multicoloured world every bit as extraordinary? A swirling world of tropical fish, pint-sized penguins, and the world's largest fish. That's the whale shark, incidentally, which trawls perpetually, mouth open wide, filtering the depths for its diet of microscopic plankton.

This innocuous beast is not to be confused with some of the other finned creatures of the deep. The Galapagos teems with silvery hammerheads, which are easy to identify by their cartoonish T-silhouettes, as well as slim, agile Silky sharks. Then there's the thick-bodied Galapagos member of the family, which means business, ranking number three on the world's-most-dangerous list. Fortunately for those of us about to get down deep, the cold waters of the Humboldt current provide them with a constant all-you-can-eat fish banquet, so they couldn't care less about human snacks in neoprene. Which is good news for this keen-as-Cousteau diver, since I also happen to have a bit of a shark phobia. Reassured somewhat, I've travelled 10,000km from England to plunge into the Pacific and meet the beasts. No cage provided.

There were iguanas basking on the Tarmac as the plane touched down on the island of San Cristobal, but I was busy trying to guess who, among the passengers, might also be in possession of dismemberment insurance. My co-travellers, it turned out, had one thing in common as well as diving: macho dive slogans on their T-shirts. It was certainly T-shirt weather by the time we reached the Pacific. The sun was merciless but the ocean was calm as we clambered over dozing sea lions to reach our dinghy (or panga, its name across the islands.) Out in the harbour, our floating home for the next week lay at anchor: 28m of polished fibreglass and chrome, the Aggressor II wouldn't shame a billionaire cruising the French Riviera, with her whirlpool tub on the bow and her immaculately uniformed crew. But the swanky-looking live-aboard was built for tougher challenges: strong Pacific swells between here and the far-flung specks to the north. Other tour boats rarely glimpse the forbidding-sounding Wolf and Darwin Islands, 'Where the really big stuff lives,' in the words of a grinning Nelson. A stocky, cheery Ecuadorian, he and Walter will be our two sub-aquatic escorts, accompanying us on every dive and keeping us out of trouble. They've got a high-tech underwater camera and promise to take lots of impress-the-mates-back-home shots. Here's hoping they won't end up treasured mementos of life before dismemberment.

There's no time for nerves, for we're straight into the water. The 'wet' deck is bristling with tanks and equipment required for the check-out dive, a chance to prove to the others you're not a complete wimp beneath the waves. The Galapagos aren't prime territory for beginners - with cyclonic currents, most people need a degree of experience under their weight-belt before taking the plunge. That said, you don't need to be an expert. You just need a thick rubber suit, a special hood and a pair of sturdy gloves. There's very little of the Ursula Andress about the look, it must be said.

The Aggressor glides into a sheltered cove and it's time to don neoprene wetsuits, into which we contort ourselves like a pack of yoga enthusiasts. The blue water is particularly chilly, fed by the currents that sweep up from the Antarctic, but these suits are designed to keep you warm as toast. 'Yummy yellow? That's brave,' says Hamish, a fearless Australian, as I pull on my canary-coloured hood. 'Sharks love yellow, but I'm sure that's just a tall story,' he adds, not entirely convincingly.

Walter flippers ahead of us, down to a sandy sea-bed scattered with glossy black urchins. I'm trying to avoid skewering my knee on a particularly vicious-looking specimen when he begins signalling vigorously, which looks like sign language for 'behind you!'. Pirouetting, I glimpse some sort of rocket-propelled boulder racing from the gloom. It's a sea-lion pup, its silky fur rippling as it makes its fly-by. Suddenly dozens more dive-bomb us from the rocks above, swerving away at the very last minute, just centimetres from our masks. It's a one-sided game of chicken. Clumsy on land, the sea lions move like torpedoes in the water, somersaulting and nibbling our fins, imitating us with long streams of bubbles. They scatter when an enormous bull male, with telltale bulbous head, swims up to check on his harem, baring alarmingly sharp-looking teeth and eyeing us grouchily. We leave him to his ladies and swim back to the boat, where crew member Francisco is waiting with mugs of steaming hot chocolate and fluffy towels. 'It only takes one good dive to change your mood for a month,' reckons Hamish, leaning over as we peel off our gear. And suddenly I'm not scared.

That night we departed under a full moon for the long dark voyage north to Darwin and Wolf Islands, the star attractions of the trip. The mood changed, and nervous anticipation seemed to crackle through conversations like an electric charge as we settled into the main cabin for one of our nightly three-course dinners. I noticed the wine being rationed carefully, which was a good thing, since sharks and hangovers don't mix. Besides, it didn't suppress talk among what was a diverse bunch. There was an effervescent blonde dive instructor on something of a busman's holiday, a Japanese wildlife documentary-maker, a mother-and-daughter combo from North Carolina with matching Southern drawls, a boisterous beer-drinking Aussie and a grown-up gapper on a break from a high-powered IT career. All were united in the same mission: an up-close-and-personal encounter with schooling sharks.

I'm woken at 6.30am by the breakfast bell. The sea is mirror-still, reflecting the pink dawn. 'El lago Pacifico' (the Pacific lake), murmurs Nelson, pointing to the horizon, where Wolf Island juts from the cobalt water, a bleak grey hulk of a rock. Guano-spattered cliffs render the island utterly inaccessible, and even if you were mad enough to want to scramble ashore, you'd be breaking the law, as it's a protected National Park. Whirling Frigate birds and Nazca boobies ride the air currents, swooping and diving at our approach. It's about as far as you can get from humanity. Around these parts, nature rules the roost.

For centuries, the only humans to set foot on the Galapagos were buccaneers and whalers, who stopped off to stock up on giant tortoises. Stacked in the hold they could survive on air for a year, providing a valuable source of fresh meat during long voyages. Now around 18,000 people make a livelihood from fishing and tourism on the main islands of Santa Cruz, San Cristobal and Isabela. Eco-tourism means more visitors every year, yet the Galapagos still have an edge-of-the-world wildness to them. Theirs is a raw beauty - barren, lava-encrusted slopes on which scrubby vegetation grows. This comes as a surprise to some visitors, who arrive expecting lush tropicality. Up here at Wolf Island, there's not a soul to be seen, just navy-blue Pacific to the horizon.

The loud exhalations of a fin whale and her calf interrupt the dive briefing on the top deck - the leviathans circle the boat curiously as Walter fleshes out unnerving scenarios. There's something alarming about his variation on the conditional tense: instead of 'if you get swept out to sea, turn on your emergency radio beacon', we are told 'when you get swept out to sea…', 'when you get carried off in a current…' and 'when you run low on air…'. By the end, I am clutching my retractable emergency flag and radio beacon as if my life depends on them. Which it does.

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