How are cenotes formed?

1. A peninsula made of limestone

The Yucatán Peninsula is a flat platform of porous limestone that formed on an ancient sea floor from shells and coral. It has almost no surface rivers: rain soaks straight into the ground.

2. Rain dissolves the rock

Rainwater picks up carbon dioxide from the air and soil, forming a weak carbonic acid. Over thousands of years it dissolves limestone along cracks, a process geologists call karst formation.

3. Underground rivers and caves

The dissolved channels grow into huge flooded cave systems. During ice ages, sea level was much lower, so many caves were dry and stalactites and stalagmites grew inside them. When the sea rose again, the caves flooded.

4. The roof collapses

As the cave roof thins, it eventually falls in. Depending on how much collapses, you get a cave cenote, a semi-open cenote or a fully open cenote. Over very long periods, an open cenote can fill with sediment and become a lagoon.

Why there are so many in Yucatán

• Thick, soluble limestone everywhere
• Lots of tropical rain and no surface rivers
• Sea-level changes that flooded dry caves
• Fractured rock around the Chicxulub crater rim

Did the asteroid impact create the cenotes?

No. Geologists at Northwestern University point out that similar karst features exist around the world and that cenotes formed through slow dissolution long after the Chicxulub impact. The buried crater only influenced where a ring of cenotes lines up in Yucatán state.

Cenotes around the world

Water-filled sinkholes also exist in Florida, the Bahamas, Belize and elsewhere, but the word cenote is used mainly for the Yucatán Peninsula, where there are thousands of them.

The role of the ice age

During the last ice age sea level was about 120 m lower, so the caves were dry. Dripping water built stalactites and stalagmites. When the ice melted, the sea rose, groundwater rose with it, and the caves flooded, which is why divers see formations underwater today.

How long it takes

Limestone dissolves very slowly, so a cave can take many thousands of years to form. The final collapse that opens a cenote can happen suddenly, in a single moment.

Fresh water, salt water and mixing

Near the coast, the mixing zone where fresh water meets seawater is especially good at dissolving limestone. That helps explain why huge cave systems like Sac Actun formed near the Caribbean coast.

Forming today

• New collapses still happen in the Yucatan, sometimes under roads or fields.
• Human pumping of groundwater and construction can trigger collapses (reported).
• Over long time scales, open cenotes slowly fill with sediment and become shallow.

Step 1: an ancient reef becomes limestone

Much of the Yucatán was once a shallow tropical sea. Shells and coral piled up for millions of years and hardened into a thick limestone shelf, which later rose above sea level.

Step 2: rain dissolves the rock

Rainwater picks up carbon dioxide from the air and soil, becoming a weak acid. It seeps through cracks and slowly dissolves the limestone, opening tunnels and caves. Geologists call this karst.

Step 3: ice ages drain the caves

During the ice ages sea levels were far lower, so many caves were dry. Dripping water then built stalactites and stalagmites, which can only form in air. Seeing them underwater today proves the caves were once dry.

Step 4: roofs collapse and the sea returns

As sea levels rose again, the caves flooded with fresh and salt water. Where a cave roof grew thin, it collapsed and opened a cenote to the sky. Each stage of collapse gives a different type: cave, semi-open or open.

Is the process still happening?

Yes, very slowly. Rock still dissolves and roofs can still collapse, which is why new cenotes are occasionally discovered, sometimes under roads or fields.