'It was like going back a million years'

A decade ago Thailand's Phang Nga Bay was unchartered territory ÿ then explorers stumbled upon a hidden beach within its island caves.

The islands loomed up from the sea in the distance, like a crowded Manhattan skyline. From our boat, which had set out from Ao Por pier, north-east of Phuket Island, they looked like ghostly, misshaped forms in flat, monotone grey, two shades darker than the sky that hung above them like day-old bath water. In front stretched a dirty shag pile of wind-chopped water.

As the boat chugged deeper into Phang Nga Bay, colour began to seep into the distant cardboard cutouts. Trees, a violent green, oozed puss-like from steep cliffs, a scraggy mass of cream and black. In places, where the water tickled, the rocks parted to reveal a flash of golden beach.

But there was more to the islands than their good looks. In 1989, John Gray, an American, Welshman Simon Warren and Thai Soonthorn Sagulsan canoed into the mouth of a cave and, at the end of a long, dark tunnel, emerged blinking into a vast, open-air lagoon at the centre of the island. "It was like going back in time a million years," Simon had told me. There were giant fruit bats hanging from the cliffs, and monkeys. We must have been the first Europeans in there."

The men quickly discovered that timing was critical. The area has the third highest tidal range in the world (10 feet), making the caves only accessible at certain periods of the day, some only at full moon. Six months later, they formed Sea Canoe Thailand, offering trips to around a dozen of the bay's 150-odd islands.

After a safety talk, we were given life jackets and climbed off the boat into a yellow inflatable canoe. Our Thai guide, Shore, sat at the back and paddled towards Phanak island. We slipped into the mouth of a cave, and, once the light had been swallowed, Shore shone a torch on to the rocks above us, illuminating a Disney-style, twinkling mass of stalactites that hung suspended like a frozen fountain.

Paddling towards a jigsaw piece of light in the distance, we emerged back into daylight. We were in the centre of the island on a silent pond, the colour of green marble dusty with footprints. Out of the cliffs, which climbed to 100 feet around us, skinny trees grew at 90-degree angles, craning towards the sunlight. In one corner of the lagoon, a mangrove tree stretched its spaghetti roots into the water. Cicadas made their din in surround sound, resembling fishing reels being turned at deafening speed. We drifted in awe, taking in the stillness.

Shore paddled back through the tunnel and we headed for the next island, Nok Koom. On the way, we dragged our feet in the warm water while Shore sang us a Thai folk song. A kingfisher skidded past, giving us a flash of his ginger belly. The canoe squeaked perilously as it squeezed its way through a narrow tunnel, accompanied by our prayers that it wouldn't pop. Once inside, we glided into a cave, where torchlight revealed a stalactite the shape of a thundering organ. We paddled round the corner and there, in the darkness, stretched a tiny beach, which had never been touched by sunlight. The cave echoed with the loud belching of water being forced in and out of a hole. The canoe glided on to the beach, and we stood on the sharp sand, clutching on to each other in the utter blackness, scared but thrilled with it. Shore shone the torch on to the ceiling, which was a grotesque and fatty intestine of pink and white swirls, on to which the fruit bats hung like giant, leathery full stops.

After Thai coconut soup and numerous other traditional dishes, cooked on board the main boat, we paddled under a sooty sky to our last island, passing copper-, black- and grey-streaked cliffs and diving swallows. Once inside the tunnel, we chose the smallest entrance to the lagoon, which meant having to lie completely flat.

With Shore constantly reminding us to keep our heads down, we passed underneath oyster-encrusted rocks resembling twisted metal, literally inches away from our noses. It was the only time I've ever wished for a smaller chest. By the time we had finished exploring the lagoon, the water level had risen, and Shore let out air from the canoe so that we were low enough in the water to pass back into the tunnel again.

As we headed towards the main boat, our canoe, which by now was part-filled with water it was so low-slung, resembled a flabby bathtub. Shore started singing a Thai love song, as the rain plopped and we trailed our legs in the warm sea.

The writer stayed at the Evason Spa and Resort in Phuket, which, as well as day trips, now offers overnight sea canoe packages, which include a two- or three-day kayak camp, with two nights back at the resort. Prices start at baht 11,650 (£185) per person. For information, call Six Senses on 0151-486 4500. For resort reservations only, call Cresta Worldwide on 0870 161 0930