Gran Cenote is a pair of pools a few kilometres from Tulum, on Mexico's Yucatán coast. The water is clear enough that pale limestone blocks several metres down look close enough to touch. You slide into the open basin under the canopy, where freshwater turtles paddle unhurried between the lily pads and silver fish scatter through the shafts of light. After the jungle heat the water is cool and faintly mineral. A low arch of rock joins that basin to a second pool behind it, roofed in stone. The short swim through that arch is the best moment here: daylight behind you, cool dark ahead, the ceiling dropping to within arm's reach and the water turning from green to deep blue as the light gives out. In the roofed half, stalactites hang down to meet their own reflections and your breath comes back off the rock.
The two pools are one sinkhole caught at two ages. Cenotes are sorted by how far the roof has fallen in: fully caved over, a half-open cavern, or a pool wide open to the sky. Here the shaded end and the sunlit end sit side by side, two stages of the same slow collapse. What you are floating on is the top of something far larger. These pools are a public doorway into the Sistema Sac Actun. Divers working out of them helped prove in 2018 that Sac Actun and Dos Ojos are one cave — at the time the longest flooded cave known, some 347 kilometres of passages running under the coast. The tunnels were dry caves before the sea rose, which is why they hold bones. In a pit deep inside called Hoyo Negro, divers found the oldest and most complete human skeleton in the Americas — a teenage girl who fell to her death twelve or thirteen thousand years ago. She lay there among mastodons, giant sloths and sabre-toothed cats that had dropped into the same trap.