A layered liquid world
The Yucatán Peninsula is a flat expanse of limestone, riddled with thousands of sinkholes called cenotes. These are openings to a vast network of flooded caves. From the surface, they appear as clear, isolated pools, but they are outlets for a subterranean estuary where fresh and salt water meet. Rainwater, filtered through the limestone, forms a lens of fresh water that floats on top of denser, saltier water that has pushed inland from the Caribbean Sea through the porous rock.
The boundary where these two bodies of water meet is called a halocline. It is not a gentle mix, but a distinct layer, often visible to divers as a shimmering, distorted zone that looks like an underwater mirage. In many cenotes, this layer is found at depths between 10 and 30 meters. Here, the salinity and density of the water change abruptly, creating a physical and chemical environment that traps organic material and hosts a specialized microbial world.
The sulfide cloud engine
The halocline is a zone of intense microbial activity. Organic matter, such as leaves and branches from the surrounding jungle, falls into the cenote and sinks. When it reaches the dense saltwater of the halocline, its descent slows or stops, and it begins to decompose. This decomposition in the low-oxygen conditions feeds a community of microorganisms.
The dominant process is driven by sulfate-reducing bacteria (SRB). These microbes, including members of the Deltaproteobacteria and Bacillota groups, thrive in the anoxic saltwater layer. They perform a type of anaerobic respiration, using the abundant sulfate from the saltwater and the carbon from the decaying plant matter for energy. The primary waste product of this process is hydrogen sulfide (H₂S) gas.
This hydrogen sulfide accumulates in the halocline, often creating a thick, milky-white cloud that can be several feet thick. Divers in cenotes like Angelita describe the experience of descending through clear water into this dense cloud, which looks like a river or a fog bank flowing through the middle of the water column. The H₂S cloud is so opaque it can create a "false bottom" illusion, with dead tree branches piercing through it from the anoxic depths below. This entire system is a chemoautotrophic food web, where life is sustained by chemical energy rather than sunlight.