In Yucatán the cenotes trace the rim of a crater.

Start with what you see. The state covers 39,524 square kilometres and is, apart from the low Puuc hills in the south, absolutely flat: Mérida, the capital, stands ten metres above sea level. No valleys, no watercourses, no bridges. Among its 2,320,898 inhabitants at the 2020 census there is not one town founded beside a river, because there are no rivers.

01 A slab of limestone that dissolves

The bedrock is limestone, and limestone has one property that decides everything: slightly acidic rainwater dissolves it. Instead of running over the surface, water filters down, opens conduits and ends up forming a network of water-filled galleries and caverns beneath the ground.

The result is karst taken to an extreme: a dry plain on the outside and a complete hydraulic system on the inside. The state's entire water supply, and its entire settlement history, depends on where that system can be reached from above.

02 A cenote is a roof that fell in

That is where the cenote comes in. The word comes from Maya and names exactly what it is: the point where the vault of one of those caverns has collapsed and left the water in view. Not a spring and not a lake: a window.

Which is why they take such different forms, and it is worth knowing before choosing which to visit. Closed ones are caverns with the roof intact, reached by a staircase, with stalactites. Semi-open ones have a partial gap through which light and roots come in. Open ones are shafts under bare sky, with vertical walls and vegetation leaning over the edge. It is the same formation at three different moments of its life.

And most look nothing like the photographs: they are mouths a couple of metres across out in the scrub, with water a few metres down, no infrastructure and no name on any map.

03 The ring of cenotes

Here is the finding that turns the state's geography into something else. In north-western Yucatán the cenotes are not scattered: they concentrate in a semicircular band about 180 kilometres in diameter, centred roughly on the coastal locality of Chicxulub Puerto.

That band coincides with a concentric ring of a structure buried beneath the platform's sediments: the Chicxulub crater, the impact site of the asteroid associated with the end-Cretaceous extinction some 66 million years ago. The crater is about 180 kilometres across and was identified in 1978 by the geophysicist Glen Penfield, who, working with magnetic and gravity data for an oil company, found a bullseye of that size in otherwise uniform ground.

The connection between the two is hydrogeological. The crater rim left a structural discontinuity beneath the later limestone, and the zone enclosed by that semicircular belt has high permeability, with groundwater flowing from the south-east of the peninsula towards the north-west. Where the rock is more fractured it dissolves sooner; where it dissolves sooner the roof falls in; where the roof falls in, there is a cenote.

04 What that means on the ground

Put plainly: in one part of Yucatán you can drive the outline of an impact crater in a day without once seeing the crater. What you see is villages, maize plots and water holes strung along an arc nobody drew.

The ring of cenotes appears on Mexico's tentative list with UNESCO, among the sites the country proposes as possible world heritage. And it also works as an explanation of human settlement: Maya sites went where there was access to water, and access to water was where the roof had already fallen.

That is the physical logic of the state, and it is rare for it to be this clean. A plain with no rivers, a water system that is entirely underground, and a map of windows onto that system whose distribution was decided by an asteroid.