Twenty kilometres north of Tulum on the road to Playa del Carmen, Cenote Dos Ojos — "two eyes" — consists of two large sinkholes connected underwater by a 400-metre passage. The two pools sit beneath an enormous limestone cavern roof, and the visibility is famously good: snorkellers can see five metres down to the bottom, where divers move silently through the gallery below.
Snorkelling is straightforward from a sloping entry, and a basic guided line keeps you above the dive zone. Certified cave divers can do guided tours through the Bat Cave passage and into deeper sections of Sistema Sac Actun, the underwater cave system to which Dos Ojos belongs. The site has a small restaurant and changing rooms; entry includes life jackets. Aim for the first hour after opening to beat the day-trippers.