The Scar of Extinction
Some 66 million years ago, an asteroid approximately 10 kilometers (6 miles) wide struck the shallow seas of what is now the Yucatán Peninsula. The impactor, traveling at an immense velocity, released energy equivalent to billions of atomic bombs, creating the Chicxulub crater. This catastrophic event triggered the Cretaceous-Paleogene (K-Pg) extinction, a mass extinction that eliminated about 75% of all plant and animal species on Earth, including all non-avian dinosaurs.
The immediate effects were apocalyptic. The impact generated a transient cavity 100 kilometers wide and 30 kilometers deep, which then collapsed to form a crater structure estimated to be 180 to 200 kilometers (110 to 120 miles) in diameter. Hundreds of billions of tons of sulfur from vaporized rock were ejected into the atmosphere, blocking sunlight and causing a prolonged "impact winter" that halted photosynthesis. The crater is now buried under nearly a kilometer of Cenozoic sedimentary rock, its existence first confirmed by geophysicists Antonio Camargo and Glen Penfield in the late 1970s while searching for oil. The primary evidence linking this specific crater to the global extinction event is a thin layer of clay found worldwide at the K-Pg boundary, which contains abnormally high levels of iridium, an element rare on Earth's surface but common in asteroids.
A Circle of Life from a Ring of Death
Today, the only visible surface expression of this immense, buried crater is a geologic marvel known as the Ring of Cenotes. This feature is a nearly perfect semicircle of sinkholes, or cenotes, with a diameter of about 180 kilometers. The formation of this ring is a direct consequence of the impact's underground structure. The crater's rim is composed of fractured and porous limestone. Over millions of years, rainwater seeping into the ground has preferentially dissolved this weakened rock, creating a vast network of underground caves and river systems that follow the circular fault lines of the crater rim. Where the roofs of these subterranean caverns have collapsed, cenotes form.
This ring dictates the geohydrology of the northern Yucatán Peninsula. Groundwater flows towards the center of the crater and is then diverted outwards along this highly permeable ring, creating a concentrated zone of dissolution and cenote formation. This unique hydrological system provides the main source of fresh water in a region that has no surface rivers or lakes. For the ancient Maya civilization, these cenotes were important water sources but were also considered sacred portals to the underworld, Xibalba. Archaeological findings within cenotes include offerings of pottery, jewelry, and even human remains, showing their deep spiritual importance.
The isolated and dark environments of the cenotes have given rise to unique ecosystems. They have a number of endemic and specially adapted species, known as stygobites. These include cave-dwelling creatures such as the blind cave fish (Typhliasina pearsei), the blind swamp eel (Ophisternon infernale), and the Mexican blind brotula (Ogilbia persei). Other common residents are cenote catfish (Rhamdia guatemalensis) and various species of guppies and mojarras. The water within many cenotes is stratified, with a layer of fresh water at the surface overlying a denser layer of saltwater below, creating distinct habitats for different organisms.