But the town, while good for a round of people-watching, is not what I've come to see. I have returned for the ruins and the biosphere and, just above it, the near-perfect beach along Boca Paila Road. (This thin strip of peninsula has a twenty-year tradition of offering cabana and casita accommodations in an atmosphere of bohemian laissez-faire.) I also hope to explore a bit more inland, driving a few of the skinny jungle roads through smaller Mayan villages and around the old colonial city of Valladolid. And I want to learn more about the region's frail, unique ecosystem—a wonder that unfolds its bizarre, interlocked workings before your eyes, and whose preservation merits serious planning.

The caves and the modest wildlife exhibit at Aktun Chen Natural Park are easily reached on my first full day—a brief drive down a gravel path into the jungle off Highway 307, thirty-five minutes south of Playa. But at this quiet, lightly drizzling hour, the place has the slightly fantastic air of a separate world. KEEP CARS LOCKED, reads a sign as I pull into the parking lot; MONKEYS AT PLAY. A few minutes later, a guide is pointing a group of us visitors toward an ocelot. The site's caretakers discovered the boldly spotted cat after it had spent weeks killing the chickens they raise to feed the rattlesnakes and yellow-beard and coral snakes on display in glass cases. Uphill from the snakes, the cat lounges on a bench in its large cage, amber-eyed and serenely mysterious.

With what strikes me as a very Mexican, macabre sense of humor (it is part of his shtick to warn visitors about lurking jaguars), the guide then walks us through the backstory of what we're about to see. Many millions of years ago, the Yucatán Peninsula rose up out of the sea, formed of what until then had been coral reef. The result is land that is basically a brittle limestone shelf. It is extremely rich in calcium and magnesium and generally poor in other nutrients. This northern portion of the Yucatán must survive without anything like what we traditionally understand as a river. Instead, the peninsula depends upon freshwater that flows down from the highlands into an elaborate network of underground channels.

The guide leads us on a six-hundred-yard walk through a series of atmospherically lit underground chambers, some dry, some with a planked path over a floor pooled with water. From above, tender tree-root tendrils poke with touching persistence through the caves' roofs in search of freshwater. Stalactites also hang from the ceiling; where stalagmites rise up from the ground, they join with one another in a crusty pillar. To demonstrate the water's high calcium content, the guide touches his nose, then bends down to dip his finger in a shallow pool. The oil on his skin makes the water fizz. "Like Tums," he jokes.

He leads us farther on, past a family of mosquito bats roosting on the ceiling and an ancient fossil of a spiral-shelled conch on the wall. Impressively, he claims to be able to read the terrain above—the trees, and the dips and swells in ground elevation—from the roots and the dripping water and the encrusted minerals below. The final chamber is a huge, craggy bowl of beautiful bright-blue water, the sight of which causes the group to hush, and turns us for a second into delighted children.