The Sistema Sac Actun

Sistema Sac Actun, or white cave system, is considered one of the longest surveyed underwater cave system in the world. Located in the State of Quintana Roo in Mexico's Yucatan Peninsula, it runs along the Caribbean coast and has passages west and north of Tulum, the site of Pre-Columbia Mayan ruins. Measuring over a hundred and nine miles, it is the second longest underwater cave system, after Sistema Ox Bel Ha, or "three paths of water", which is also located in Quitana Roo and is just four-tenths of a mile longer than Sistema Sac Actun. The caves are always filled with water at a seventy-six degree Fahrenheit temperature. Layers of water, with freshwater on top of salt water, flood the caves, most of which become trapped in the rock. Throughout the Yucatan Peninsula are sinkholes through which divers can access the underground water caves.

The underground water-filled caves were formed millions of years ago and are a remarkable natural resource. This system maintained the Mayan's need for water for centuries and is still the main source of fresh water for the region.