What is a Cenote?

During the Mesozoic era, the expanse of land from the south central region of Texas to the southern-most extent of Belize into Guatemala was covered by ocean waters. For millions of years, reefs were built by sea corals and animals. These ancient marine deposits gave rise to a thick layer of limestone which was variously uplifted, distorted, and compressed to form the Sierra Madre Oriental, the coastal ranges of mountains, and the coastal plains. The last region to emerge from the ocean was the Yucatan Peninsula. Rain mixing with carbon dioxide from vegetation then falling on limestone creates a weak carbonic acid which over millions of years dissolved the rock along the paths of least resistance, giving rise to solution caves.

It is believed that an asteroid impacted with the planet somewhere near Merida on the Yucatan, causing an ice age, the death of the dinosaurs, and a drop in sea level. During this ice age, highly decorated caves were formed in the Yucatan Peninsula. The formations were created by millions of years of dripping action that left behind mineral deposits, forming stalactites, stalagmites, columns and various other types of speleothems. As the global climate returned to homeostasis, the water levels rose and the caves became flooded.

For millions of years, water filtering through this limestone produced vast stores of water reservoirs in the form of underground rivers. Cenotes yield surface acess to these subterranean passages. Water pouring forth from the base of the mountains and from natural springs are the nacimientos, or birth places, of these water-filled caves.

Some cenotes were formed from collapses in roofs of large, hollowed out passageways. Others form along base rock faults, some, nearer to the shoreline, carve river-like passageways for the last few hundred meters.

The water in the cenotes is normally exceptionally clear after having been filtered through the limestone rock substrate. The water is fresh, at times slightly brackish, until we dive to the depth of the halocline. At this point, we reach pure salt water, and there is a lack of current.