The hydrogen sulfide cloud in cenotes

What the cloud is

In some deep cenotes, divers swim down into what looks like a foggy river or a layer of smoke. It is a cloud of hydrogen sulfide gas dissolved in the water, made by bacteria breaking down leaves and wood where there is no oxygen.

How it forms

• Leaves and branches fall into the cenote and sink.
• They collect on the halocline, where heavier saltwater sits under freshwater.
• With no oxygen there, bacteria decompose them and release hydrogen sulfide.
• The gas stays trapped at that density boundary, creating a visible milky layer.

Cenote Angelita: the famous 'underwater river'

Angelita, south of Tulum, is reported to be about 60 m deep with the cloud at around 30 m. Dead tree branches poke out of the layer, making it look like a river with a riverbank. Divers pass through the cloud into clear saltwater below. Details on depth and access are on our Cenote Angelita profile.

The Pit

The Pit, in the Dos Ojos park, is a deep vertical cenote where divers also pass a hydrogen sulfide layer and a halocline, with famous light beams above (reported). See The Pit profile.

What it feels like

• Visibility drops to almost nothing inside the cloud for a few metres.
• Some divers report a rotten-egg taste or smell around the regulator.
• Below it the water is often very clear and saltier.

Is it dangerous?

• It is a diving feature at depth; swimmers and snorkelers never reach it.
• Only certified divers (usually deep or advanced training) should go, with a guide.
• The main risks are depth, buoyancy control and losing orientation in low visibility, not the gas itself (reported).

Hydrogen sulfide vs halocline

They are different things. The halocline is the boundary between fresh and salt water, which looks blurry like oil. The hydrogen sulfide cloud is a layer of gas-rich water that often sits right on top of the halocline.

Hells Bells at Cenote Zapote

At Cenote Zapote near Puerto Morelos, bell-shaped rock formations hang from the walls at about 29 to 35 m, just above the hydrogen sulfide layer. Scientists think the cloud, the water chemistry and microbes all help these rare formations grow, so they are found almost nowhere else.

Photographing the cloud

Divers light the cloud from the side or from above so it glows like fog, and often frame a tree branch or another diver against it. Staying above the cloud protects your camera and your buoyancy, and keeps the layer from being stirred up.