The Quintana Roo cenotes

The Yucatán peninsula is a slab of young, porous limestone with no rivers on it at all. Rain sinks straight through, and over the last few million years — repeatedly drowned and drained as sea level rose and fell through the ice ages — it has dissolved the largest surveyed underwater cave systems on Earth beneath the coastal strip between Playa del Carmen and Tulum.

A cenote (from Yucatec Maya dzonot) is simply a place where the roof has fallen in and you can get at the water. They were the peninsula's only water supply, which is why Maya settlement follows them, and why so many hold pottery, bones and — in the Tulum caves — some of the oldest human remains in the Americas.

Two features define the diving. The first is the halocline: fresh rainwater floats on salt water pushed inland from the sea, and the boundary between them is a shimmering, oily-looking layer that blurs everything until you pass through it. Depth of the halocline is a fingerprint of each cave — 16 m at Corazón, 19 m at Tortuga. The second is decoration: because these passages were dry caves during the glacial low stands, they are full of stalactites, stalagmites, columns and helictites that only form in air.

Systems get connected, and then renamed. Sistema Sac Actun absorbed Nohoch Nah Chich in 2007 and Dos Ojos in January 2018, at which point it became the longest underwater cave in the world at over 370 km. Sistema Ox Bel Ha, further south, is the other giant.

The individual site files give each cenote's system, surveyed length, depth and what is in it.