It is hard to think of any natural environment that can be more easily destroyed than that of a cave,"" says Roger Brucker, director of the Cave Research Foundation in a foreword that explains the exclusionary attitude of today's spelunkers. Laycock takes the cue and treads lightly, leading a low-keyed, peripatetic tour of the world's great caves--covering their discovery, exploration, uses, and ecology. The amateur carets one meets are a dedicated, methodical bunch who strive to avoid ""thrills,"" and there are repeated warnings of the damage vandals and litterers can and have done to fragile underground formations. The concern is ever-present but never off-putting, and there's a rich vein of lore and anecdotes to be mined (the misguided attempt to cure tuberculosis patients in the dank air of Mammoth Cave is one of the sadder curiosities). If Laycock doesn't lead would-be explorers very far beneath the surface, he is a well-equipped, undemanding guide for first-time visitors.