There are no rivers on the surface of the Yucatán Peninsula. None. Every drop of rain that falls goes straight down through the rock — and what it built underneath is the longest flooded cave system on the planet. How cenotes work is the key to understanding everything else here, including why the lagoons of Sian Ka’an are clear.
A Peninsula Made of Old Sea Floor
The Yucatán is a slab of limestone — compacted marine sediment lifted above sea level. Limestone is porous and soluble: rainwater, made slightly acidic by carbon dioxide, dissolves it. Over millions of years this created a honeycomb of caves and passages beneath the entire peninsula.
How a Cenote Forms
During ice ages, sea level dropped and caves drained and grew. When sea level rose again the caves flooded. Where a cave roof eventually collapsed, the water below was exposed: that opening is a cenote, from the Maya ts’onot. Some are open pools, some are half-domed, some are almost fully enclosed with a small hole above.
The Ring of Cenotes and the Asteroid
Many cenotes across the northern Yucatán sit in a striking semicircular arc. That arc traces the buried rim of the Chicxulub crater, left by the asteroid impact linked to the extinction of the dinosaurs 66 million years ago. The fractured rock along the rim dissolved preferentially — so the geography of an extinction event still decides where you can swim today.
Why the Water Is So Clear
It has been filtered through hundreds of metres of rock, and it carries almost no suspended sediment because there are no surface rivers to stir it. The same filtration is why the Sian Ka’an lagoons are transparent where they should be muddy wetland.
Why It Is Also Fragile
Filtration removes particles, not chemicals. Fertiliser, sewage and sunscreen enter the aquifer and travel — eventually surfacing in cenotes, lagoons and finally the reef. Everything is connected underground, which is why the rules about biodegradable products are not a formality.
What the Maya Understood
Cenotes were the only reliable water source, so settlements grew around them — and they were also understood as entrances to Xibalba, the underworld. Offerings and remains have been recovered from many. Practical and sacred at once, like the ceiba.
Meet the Community Behind the Tour
In just two minutes, discover who we are — the Maya families and cooperative members who make every experience authentic and regenerative:
Frequently Asked Questions
How are cenotes formed?
Rainwater dissolves porous limestone over millennia, creating caves. When a cave roof collapses, the flooded passage below is exposed — that opening is a cenote.
Why are there no rivers in the Yucatán?
The limestone is so porous that rainfall drains straight down into the aquifer instead of running across the surface.
Are cenotes connected to the sea?
Yes. The underground system flows toward the coast, surfacing in lagoons and eventually reaching the Mesoamerican Reef, which is why pollution inland affects the reef.
See Where the Underground River Surfaces
Book your day in the lagoons with the Maya cooperative.
Questions? Contact us at reservacionesctsk@siankaantours.com.mx or WhatsApp +52 984 114 0750