A descent into the clouds
Cenote Angelita, meaning "Little Angel," is a deep, cylindrical sinkhole located about 17 kilometers south of Tulum. From the surface, it is a circular pool in the jungle. The dive begins with a descent through clear freshwater. For the first 10 meters, the water can be hazy with organic material from the surrounding vegetation. Below this, the visibility improves dramatically in water that maintains a temperature of about 25°C (77°F) year-round.
At a depth of approximately 30 meters (98-100 feet), divers encounter the cenote's main feature: a dense, milky-white cloud of hydrogen sulfide. This layer, which is several meters thick, separates the upper layer of freshwater from the saltwater below. To a viewer from above, the cloud looks like a river flowing through an underwater forest. Large branches and whole tree trunks, preserved in the low-oxygen environment, rise eerily out of the mist, creating an unusual environment.
The science of a stratified world
The "river" is a product of chemistry and biology within a geological formation unique to the Yucatán Peninsula. Cenotes are sinkholes formed by the collapse of porous limestone bedrock, a process that has occurred over thousands of years. This geology creates deep, water-filled shafts like Angelita, which reaches a maximum depth of around 60 meters (200 feet).
The halocline is—the boundary where the less dense freshwater from rainfall sits atop the denser saltwater that seeps in from the ocean through underground passages. Organic debris, such as leaves and branches from the jungle, falls into the cenote and sinks. As this material decomposes in the anoxic (low-oxygen) saltwater layer, sulfate-reducing bacteria produce hydrogen sulfide (H₂S) gas. This gas dissolves to form the thick, acidic cloud that becomes trapped at the halocline.
Passing through the hydrogen sulfide layer is a disorienting experience for divers; visibility drops to nearly zero for a few moments. The gas also smells of rotten eggs. Below the cloud, the water is clear again but significantly darker, as the sulfide layer blocks most of the sunlight. Diving in Cenote Angelita is considered an advanced dive, requiring at least an Advanced Open Water certification due to the depths involved.