Cenote Dos Ojos opens as two round pools side by side, the two eyes of its Spanish name, in the jungle north of Tulum. The water in them is so clear that divers routinely see more than 100 metres ahead through it. Stand on the wooden rim and you can make out the floor of the pool, the curtains of pale stalactites under the ledge, and the black mouths of tunnels leading away into the rock. Those tunnels do not stop. In 2018 divers linked the passages behind the eyes to Sistema Sac Actun. The result was a single network of more than 370 kilometres, the longest flooded cave system ever mapped, running in the dark under the jungle. All of it was dissolved out of soft Yucatán limestone by rainwater, and rainwater still filtering down through that same rock is what keeps the eyes so sharp.
Step in and the water is a cool, steady 25°C, and so clear that you seem to hang in mid-air above the stone below. Shafts of daylight drop through the two openings and stand in the blue like solid pillars, tree roots trailing down from the rim into them. Two marked routes lead deeper: the bright, shallow Barbie Line, gentle enough for a first cavern dive, and the darker Bat Cave Line, which surfaces inside a domed air pocket where hundreds of bats cling to the ceiling. Snorkellers stay in the open eyes, where the light is best anyway, and swim the short passage joining the two pools. Dos Ojos is a protected reserve now, and because this is a living cave feeding the aquifer that the whole coast drinks from, sunscreen and repellent are banned and a rinse-off shower before entry is compulsory.