Into Thailand's Lost World - When John Gray discovered collapsed island caves and pristine lagoons, he never dreamed they could be destroyed by too many visitors.

YOU'D NEVER FIND the slot between the surging sea and soaring limestone cliffs. But about ten years ago, John Gray did, and now he leads us expertly into this eerie and precarious passage. Pressed down flat against kayak decks, our noses almost scraping the sharp oyster shells that cling to the cavern roof, we inch our way through darkness and claustrophobia.

Then, having burrowed into the heart of an island, our group emerges into a primeval lagoon. Pacific reef egrets circle above, wings arched like those of the pterosaurs. Spiky cycad palms, the dominant plant life of the Jurassic Period, cling to sun-dappled cliffs as in that age of dinosaurs. Only a troop of long-tailed macaques, scampering among treetops, breaks the unearthly silence.

We have entered the lost world of Thailand's Phangnga Bay, about 400 miles south of Bangkok, where more than 40 islands rise from milky green waters. Fabulously sculpted by wind, rain, waves and currents, they are part of a great arc of limestone formations stretching from southern China to Borneo. Here in Phangnga, those natural forces have bored through karst to create labyrinths of passageways and cave chambers. When the roofs of huge caves collapse, sunlight allows plants and wildlife to flourish within these hidden gardens of Eden, which the Thais simply call hong-rooms.

Gray's green eyes still dance with boyish excitement as he points out the lagoons' wonders. But now there is a bittersweet quality to his enthusiasm for these havens he discovered. As we watch the world as it must have looked a million years ago, he turns and confesses, "I wish I had never found them."

Gray, 54, is an American activist, sea kayaker and ecotourism innovator. But his dream to save this remote bit of Thailand by opening it to only a limited number of nature-loving tourists-the essence of ecotourism-has backfired. Now his award-winning company, Sea Canoe, has a host of copycats. The solitude of the islands has been shattered forever (as many as 1,000 kayakers a day have penetrated these once- remote enclaves, driving away wildlife and causing traffic jams), and he has even received a death threat from the local mafia. In a world in which tourism has been touted as a promising way to save nature, his experiences illustrate that, once out of control, tourism can become a precarious force that can actually destroy the nature it intends to save.

The story of Gray and Sea Canoe is a kind of ecomorality play. The …