Dos Ojos is a pair of round pools in the jungle north of Tulum, on the Caribbean coast of Mexico's Yucatán, and the water in them is clear for about a hundred metres. The name means two eyes. They are holes in the roof of a flooded cave, and the tunnels behind them run into Sac Actun, the longest underwater cave system ever surveyed. None of it was cut by water you could swim in. Rain dissolved the soft limestone into a honeycomb of passages over hundreds of thousands of years while the caves stood in open air, and the stalactites grew then, in the dry. When the last ice age ended the sea rose, the groundwater came up with it, and the whole maze drowned. So the pale spikes hanging below you were made by dripping, not by the water they now hang in.
Wooden stairs and a deck are built into the rim of one eye, and the drop to the water is short. It holds a steady 25°C, fresh and cold-feeling after the heat, with tree roots trailing from the rim. You float face down over the stalactite ceiling and the black mouths of the tunnels, then swim the cool channel that joins the two pools. Around midday the sun stands high enough to send shafts of light down into the blue like columns. The Maya did not treat wells like this as swimming holes: the word cenote comes from ts'ono'ot, sacred well, and the dark water was a threshold into Xibalba, the realm of the dead, and offerings have been lifted out of these pools. Divers follow two fixed lines further in. The Barbie Line is the bright one, shallow and daylit between stalactites, the usual first cavern dive for anyone who has only dived in open water. The other rises deep in the rock into a pocket of air the flood never reached, and you break the surface in near-darkness with hundreds of bats hanging above the black water.