For our first dive we pile into a small skiff and motor to a site called "Manuelita," an islet across a narrow channel from Cocos. "This isn't beginner diving!" yells Sisinio, our dive master, as wind howls across the bucking bow and giant swells toss the skiff like a Styrofoam buoy. "Mind the currents! Watch your gauges!" We fumble to strap on tanks and fins, and then we line the edges of the boat, gripping with white knuckles. On Sisinio's cue, I place my regulator in my mouth and somersault backward over the side of the boat and into the churning ocean. I twirl and right myself beneath the thrashing foam and then fin quickly away from the surface, down away from the sunlight, and toward the tranquility of the reef.

Waterfall-laced cliffs of Cocos.

Cocos' underwater splendor lies not in dainty, aquarium-style fishies and corals—à la the Caribbean—but in monolithic, volcanic formations and majestic, ocean-going wildlife. Unlike the Caymans, where dive masters promise you "might" see "an" eagle ray, or the Bahamas, where some "naturalist" named Roy lures a couple of couch-potato sharks out of the coral with a rump roast, Cocos swarms with huge animals, every dive, au naturel.

Holy Hammerheads!

At Manuelita, house-size boulders stair-step down the sides of the islet, creating chasms, gorges, and caverns where hundreds of resident white-tipped reef sharks weave, dart, and patrol. I slip past a cadre of six-footers—who ignore me—and cling to a rocky overhang that affords a bird's-eye view of the neighborhood. To my left, a slow-moving tornado of big-eyed jacks in the thousands swirls across the boulder field. To my right, a pair of eagle rays swoops along a ridge line and dips elegantly into a bottomless gorge. A trio of dolphins spins and flips and clowns its way past a burly, hawksbill turtle that bumbles from rock to rock. The hammerheads materialize suddenly, 300 or 400 of them, turning methodical, counterclockwise circles and rhythmically arching their muscular bodies back and forth. As I run low on air and begin a leisurely ascent, I am suddenly compelled to fling open my arms wide to receive what surely must be a messenger from God—a giant Pacific manta ray with a nine-foot wingspan, soaring overhead and fading magically into the distant blue.