The illusion of a river
Cenote Angelita, or "Little Angel," is a deep sinkhole in the jungle south of Tulum. From the surface, it appears to be a typical circular pool of water, but beneath the surface lies a unique geological and biological phenomenon. Divers descending into Angelita first travel through about 30 meters (98 feet) of clear, fresh water. At that depth, they encounter what looks like a dense, swirling cloud with a strange quality.
This cloud is a thick layer of hydrogen sulfide (H₂S) that rests on the halocline, the boundary where the less dense freshwater meets the denser saltwater below. Fallen trees and other organic debris from the surrounding jungle have sunk and collected on this layer, creating the illusion of an underwater riverbed with dead trees emerging from the "mist." The hydrogen sulfide is produced by the bacterial breakdown of this organic matter in an environment without oxygen. Visibility within the cloud, which can be several meters thick, drops to nearly zero.
The chemistry of the cloud
The Yucatán Peninsula's geology is mostly porous limestone. Rainwater filters through the ground, forming extensive underground river systems that connect to the sea. In cenotes like Angelita, this freshwater sits on top of saltwater that has pushed inland from the Caribbean.
The hydrogen sulfide layer forms at the boundary between these two water types. Sulfate-reducing bacteria thrive in this anoxic zone, decomposing organic matter like leaves and branches that fall into the cenote. This decomposition releases hydrogen sulfide gas, which remains trapped at the halocline due to the density difference between the fresh and salt water. While H₂S gas smells like of rotten eggs, divers breathing compressed air do not notice it. Passing through the cloud is safe, but the mix of hydrogen sulfide and water can create sulfuric acid, so divers are advised not to clear their masks within the layer. Below the cloud, the water is dark, clear saltwater, and the cenote reaches a total depth of about 60 meters (197 feet).