Crouching on all fours, wearing hard hats and knee pads, we shuffle and drag ourselves one by one through the small earthy hollow. When we reach the other side, a large cavern spreads out before us: countless stalactites hang suspended like icicles from the roof and mineral draperies dress large chalk columns. The floor beneath our feet is uneven, rising and falling like waves, before disappearing into a body of water at the other end.

There is no known public record of the enormous limestone cave we’re in, which is at least 100 meters (about 330 ft.) long, and somewhere underneath Tulum, a municipality in the Yucatan Peninsula known for its tourist beaches and Mayan ruins. It is one of an estimated 10,000 cenotes, water-filled sinkholes formed by the collapse of li…