An aquifer shaped by an asteroid
The Yucatán Peninsula is a vast, flat platform made of porous limestone, a type of rock formed over millions of years from the accumulation of marine organisms on the sea floor. This geology, known as karst, lacks surface rivers. Instead, rainwater, which is slightly acidic, percolates through the limestone, dissolving the rock and carving out a massive network of underground rivers and caves. These water-filled caves are the cenotes.
A cataclysmic event 66 million years ago had a deep effect on this system's formation. A 10-kilometer-wide asteroid struck the Earth, creating the Chicxulub crater, an impact structure over 180 kilometers in diameter buried under the peninsula. This impact fractured the bedrock deep below the surface. These fractures created zones of weakness, influencing how water would later flow and dissolve the limestone. Today, a semicircular arc of cenotes on the surface, known as the Ring of Cenotes, precisely marks the rim of this ancient, buried crater.
The caves themselves formed during more recent ice ages, when global sea levels were much lower. With the water table dropped by 100 meters or more, the caves were dry, allowing for the slow drip of mineral-rich water to form stalactites and stalagmites. As the ice ages ended and sea levels rose, the caves flooded, preserving these formations in an underwater world.
Exploring the subterranean rivers
The Yucatán contains the longest underwater cave systems in the world. The two largest are Sistema Sac Actun (386 km) and Sistema Ox Bel Ha (541 km). Cave divers have been exploring and mapping these systems since the 1980s. In 2018, explorers proved that the then-separate Sac Actun and Dos Ojos systems were connected, creating a single mega-system that is now recognized as the second longest in the world. These networks contain hundreds of individual cenote entrances.
Diving in these caves reveals a unique hydrogeological phenomenon: the halocline. This is the boundary layer where the less dense, fresh rainwater floating on top meets the denser saltwater that seeps in from the Caribbean Sea. The difference in salinity refracts light, creating a shimmering, blurry effect that can look like a mirage or a river flowing through the water. The depth of the halocline varies, occurring from 10 to 30 meters below the surface depending on the distance from the coast.
In some deeper cenotes, such as Cenote Angelita, another layer forms beneath the halocline. Decomposing organic matter at the bottom releases hydrogen sulfide, which gets trapped between the freshwater and saltwater, forming a thick, toxic cloud that looks like an underwater riverbed.
A portal to the past
These submerged caves are geological wonders; they are records of history. During the last ice age, when the caves were dry, they acted as natural traps. Animals and even early humans fell in, and their remains were preserved when the caves later flooded.
Archaeologists have uncovered an incredible record of the past within the cenotes. In the Hoyo Negro ("Black Hole") pit within Sac Actun, divers found the nearly complete skeleton of a teenage girl named Naia, who lived between 12,000 and 13,000 years ago. Hers is one of the oldest and most complete human skeletons ever found in the Americas. Alongside her were the remains of 26 extinct Pleistocene mammals, including saber-toothed cats, giant ground sloths, and elephant-like gomphotheres. In other cenotes, fossils of prehistoric horses and a new species of giant sloth, Xibalbaonyx oviceps, have been identified. These discoveries show the fauna and human populations that inhabited the region thousands of years ago.