The underground river systems of the Yucatan flow beneath the entire peninsula. During the ice ages when the ocean levels were much lower than they are today, what was once a giant coral reef became exposed to the atmosphere and eventually became the Yucatan Peninsula. Massive cave systems were formed by gradual dissolving of the highly porous coral limestone. These caves are called "solution caves" because they were formed by the slightly acidic rainfall dissolving the alkaline limestone. Inside the caves the geological formations such as stalactites and stalagmites are a spectacular sight to see. Many of the caverns eventually collapsed and the sea levels rose partially or completely flooding the cave systems. The water table of the entire peninsula is filled with consists of seawater at sea level and freshwater 'floating' on top at varying depth depending on the distance from the sea. For instance, Cenote Zaci in Valladolid in the central Yucatan area is about 35 metres from ground level to the surface of the freshwater and probably another 30 metres below that would be the top of the saltwater layer.
It has been estimated that there are approximately 30,000 cenotes or exposed access points to these cavern and cave systems and thousands of miles of underwater cave passageways have already been explored and exploration continues in too many systems to count. Two of these cave systems have over 140 km of explored passages.
Cenotes are complexes of sinkholes and caves in the Karst geological landscape of the Yucatán. Some cenotes contain spectacular cave formations, while others are important archaeological sites, and several were considered sacred by the Mayans. A few are open to the public for swimming and diving. Of the estimated 30,000 cenotes, many of them unexplored, many are considered to be Mayan cultural and archaeological sites. Ancient fossilized remains of Camels, giant Jaguars and Mammoths are among the interesting archaeological finds in recent years. Most of these have been found by cave divers exploring underwater cave systems and some sites are now protected by INAH, the Mexican government archaeological and historical protection organisation.
Tour operators emphasize the sensitive nature of cenotes, and La Ruta de los Cenotes was first promoted as an ecotourism attraction that would offer sustainable development for the region. To promoters in Quintana Roo, it was a way to bring tourist revenue to a relatively forgotten and marginalized part of the Mayan Riviera. However, some Mexican environmentalists have criticized the construction of the eastern segment of the highway, both for the destruction of pristine forest lands and for the use of heavy equipment in sensitive areas.
Recently, experienced divers have discovered Maya artifacts upsteam of some of the sinkholes they explored dating back over 1,000 years. This has led them to conclude that the