The Yucatán aquifer

How it works

Rain sinks into the limestone and flows slowly through caves toward the coast, where it mixes with seawater.

Why it is fragile

• Thin soil means pollution reaches it quickly
• Sewage, farming and construction can contaminate it
• Sunscreen and chemicals harm cenote life

How big is it?

National Geographic describes the aquifer feeding the cenotes as covering about 64,000 square miles and being the region's only source of drinking water.

A peninsula without rivers

The Yucatán is a flat limestone platform with almost no rivers or lakes on the surface. Rain soaks straight into the porous rock, so the region's fresh water travels underground through cracks and caves. Cenotes are where the roof has collapsed and you can see that water.

Fresh water on top of salt water

Fresh rainwater is lighter than seawater, so it floats as a lens on top of salty water that seeps in from the coast. Where they meet is a halocline, the blurry layer divers see in many cenotes. The fresh layer is thinner near the coast and thicker inland.

The Ring of Cenotes

In the northwest, cenotes line up in an arc along the buried rim of the Chicxulub impact crater. The fractured rock there channels groundwater, which is why so many cenotes formed along the ring.

Threats to the aquifer

• Untreated sewage from towns and hotels seeping through thin soil.
• Pig farms and agricultural chemicals in Yucatán.
• Construction over caves, including along the Tren Maya route.
• Rising salt water as more fresh water is pumped out.

What visitors can do

• Shower and skip sunscreen before swimming.
• Use eco-friendly hotels that treat wastewater.
• Never leave rubbish near cenotes.
• Choose responsible tour and dive operators.

A freshwater lens over salt water

The aquifer is not one lake. Fresh rainwater floats as a thin layer, or lens, on top of heavier salt water that reaches far inland under the peninsula. Near the coast the fresh layer is only a few metres thick; inland it gets thicker. The boundary between the two is the halocline divers see in cenotes.

Where the water flows

Without rivers on the surface, water moves through underground cave systems toward the coast. On the Caribbean side it exits through springs and caves on the shore and even under the sea. This is why cave systems like Sac Actun are studied as water highways.

Threats from people and the sea

Studies have found sewage, medicines, personal care products and farm chemicals in the groundwater, mostly where towns and hotels grow fast. Pumping too much and rising sea levels also push salt water inland, which can spoil wells in coastal towns.

Why it matters to visitors

Everything you put in a cenote ends up in this drinking-water supply. Skipping sunscreen, using toilets on site and choosing operators that treat wastewater all help protect it.