Stalactites and stalagmites in cenotes

How they grow

Dripping water leaves tiny deposits of calcite. Stalactites hang from the ceiling, stalagmites rise from the floor, and when they meet they form columns.

Proof of ancient climate

Their presence underwater tells scientists that sea levels were once far lower, and dating them helps reconstruct past climate.

Protect them

• Never touch or stand on formations
• They take thousands of years to grow a few centimeters

How cenote caves formed first

Rain picks up carbon dioxide from the air and soil and becomes a weak carbonic acid. Over hundreds of thousands of years it dissolved passages in the Yucatán's limestone, which formed from ancient coral reefs. During the ice ages, sea level was reported to be up to about 120–130 m lower, so many of these passages were dry caves. That is when most formations grew.

Types of formations you can see

• Stalactites: icicle shapes hanging from the ceiling, built drip by drip.
• Stalagmites: mounds rising from the floor under a drip.
• Columns: where a stalactite and stalagmite join.
• Soda straws: thin hollow tubes, the first stage of a stalactite.
• Flowstone and curtains: sheets and folds where water ran down walls.
• Rimstone dams: small terraces around pools where minerals built up at the edges.

Why they are underwater now

When the ice ages ended, melting ice raised sea level and the groundwater rose with it, flooding the caves. Growth stopped, and the formations were preserved like a time capsule. Divers in systems like Sac Actun and Dos Ojos swim through rooms full of them.

Other features in cenotes

• Haloclines: a blurry layer where fresh water sits on denser salt water.
• Hydrogen sulfide clouds: a fog-like layer formed by rotting leaves in some deep cenotes.
• Fossil shells and corals in the walls, showing the rock was once a reef.

Where to see them

Cave and semi-open cenotes have the most formations above and below the water. Good examples for swimmers and snorkelers include Dos Ojos, Xkeken near Valladolid and Cenote San Ignacio near Mérida. In cave cenotes, a mask lets you see formations under the surface.

Rare formations: Hells Bells

At Cenote Zapote near Puerto Morelos, divers see hollow, bell-shaped formations up to about 2 m long hanging under water. Unlike normal stalactites, they are believed to have grown under water, helped by microbes and the water's chemistry, which makes them one of the rarest cave features in the world.