A Peninsula Made of Caves
The Yucatán Peninsula is unlike almost any other landmass on Earth. It's a flat slab of limestone, rarely more than a few dozen metres above sea level, sitting on top of one of the most extensive cave networks ever discovered. There are no mountains, no rivers, no lakes on the surface — all the water is underground, flowing through caves and channels dissolved out of the rock over millions of years.
Cenotes are the places where the roof of this underground world has collapsed, exposing the water below. They're windows into a hidden system that stretches across the entire peninsula.
Step 1: The Coral Reef (Millions of Years Ago)
The story begins in the ocean. For hundreds of millions of years, the Yucatán was underwater — a shallow tropical sea where coral reefs grew, died, and accumulated on the seafloor. Over time, the remains of these organisms — coral, shells, marine skeletons — were compressed into limestone, building up layers hundreds of metres thick.
This is why the Yucatán is made of limestone: it's the compressed remains of an ancient coral reef. And limestone has a crucial property that makes cenotes possible — it dissolves in slightly acidic water.
Step 2: The Uplift
Tectonic forces gradually lifted the Yucatán above sea level, exposing the limestone to the atmosphere. This happened in stages over millions of years, with the peninsula rising and falling with changing sea levels and geological movements. The result is a flat, low-lying platform of porous rock — the foundation for everything that follows.
Step 3: The Dissolution (Ongoing)
When rain falls on limestone, it picks up carbon dioxide from the atmosphere and soil, forming a weak carbonic acid. This slightly acidic water dissolves the calcium carbonate in the limestone as it seeps downward through cracks and fissures.
Over thousands of years, this process widens the cracks into channels, the channels into tunnels, and the tunnels into caves. The Yucatán's underground cave system — over 1,500 km of explored passages in the Sistema Sac Actún alone — was carved out by this slow, patient chemical reaction.
The formula is simple:
Rainwater + CO₂ + Limestone → Dissolved caves
The process accelerates in the tropics, where heavy rainfall and warm temperatures increase the rate of dissolution. This is why the Yucatán has more caves and cenotes than temperate limestone regions.
Step 4: The Ice Ages
During the Pleistocene ice ages (roughly 2.6 million to 12,000 years ago), global sea levels dropped as water was locked up in polar ice caps. The water table in the Yucatán dropped with it, draining the cave systems and exposing their interiors to air.
This is when the formations grew. Stalactites, stalagmites, and columns — the spectacular formations you see in cave cenotes today — can only form in air, not underwater. Water dripping from cave ceilings deposited calcium carbonate grain by grain, building formations that grew less than a millimetre per century.
When the ice ages ended and sea levels rose again, the caves flooded. The formations were submerged in water, perfectly preserved. This is why cenote divers swim past stalactites — structures that formed in air and are now frozen in time, thousands of years underwater.
Step 5: The Collapse (Creating Cenotes)
A cenote forms when the roof of an underground cave becomes too thin to support its own weight and collapses. This can happen gradually (a slow sag over centuries) or suddenly (a dramatic sinkhole appearing in a field). The result is the same: an opening in the surface that exposes the underground water.
The three types of cenotes reflect different stages of collapse:
- Cave cenotes have a small opening in an otherwise intact ceiling. These are the youngest cenotes — the collapse has just begun.
- Semi-open cenotes have a partially collapsed ceiling, with some sections open to the sky and others still roofed.
- Open cenotes have a fully collapsed ceiling. These are the oldest cenotes — the entire roof has fallen in, leaving a circular or oval sinkhole open to the sky.
Over geological time, all cenotes progress from cave to semi-open to open. The cenote you swim in today is a snapshot of a process that started millions of years ago and will continue long after.
The Chicxulub Connection
The Yucatán's cenotes have a hidden pattern. If you plot them on a map, there's a distinct ring of cenotes tracing an arc across the northwestern Yucatán — roughly from Celestún on the west coast, through Mérida, to Dzilam de Bravo on the north coast.
This ring corresponds to the buried rim of the Chicxulub impact crater — the site where a 10-kilometre asteroid struck Earth 66 million years ago, triggering the mass extinction that ended the age of the dinosaurs.
The impact fractured the limestone along the crater rim, creating zones of weakness where dissolution happens faster. Over millions of years, these fracture zones developed denser cave networks and, eventually, more cenotes. The "Ring of Cenotes" is visible on satellite imagery and has been confirmed by geological surveys.
The cenotes you swim in were shaped, indirectly, by the same event that killed the dinosaurs.
The Water Table
Today, the Yucatán's caves are partly flooded by the water table — the level below which the porous limestone is saturated with water. This water table sits a few metres below the surface in coastal areas and deeper (10–20 metres) in the interior.
The water in cenotes is part of a single, interconnected aquifer that flows slowly from the interior towards the coast, where it emerges as submarine springs in the Caribbean. This aquifer is the sole source of fresh water for the peninsula — there are no rivers or lakes to draw from.
Near the coast, seawater seeps inland through the porous limestone, creating the halocline — a boundary between fresh water (on top) and salt water (below) — that divers encounter in coastal cenotes.
Still Forming
Cenote formation is not a historical event — it's an ongoing process. New sinkholes appear in the Yucatán every year, though most are small and in unpopulated areas. The limestone is still dissolving, the caves are still growing, and roofs are still collapsing.
In geological terms, the Yucatán's cenotes are young features on a young landscape. The peninsula has been above sea level for only a few million years — a blink in geological time. Given another million years of tropical rainfall, the cave systems will expand, more roofs will collapse, and the landscape will continue its slow transformation.
Browse all cenotes and explore the results of millions of years of geology.