Cenote

A freshwater limestone sinkhole opening into flooded cave systems, famed in Mexico's Yucatan for crystal visibility, light beams, and haloclines.

A cenote is a natural sinkhole formed where the limestone roof of a flooded cave system collapses, exposing fresh groundwater. The word comes from the Mayan dzonot, and the Yucatan Peninsula in Mexico holds thousands of them, connected by some of the longest underwater cave systems on Earth, including Sac Actun at well over 300 km of mapped passage. To the Maya they were sacred wells and portals to the underworld.

Diving a cenote means gin-clear fresh water, often 50 m plus visibility, dramatic light beams stabbing through the entrance, stalactites and columns formed when the caves were dry, and in many sites a shimmering halocline where fresh water meets intruding sea water. Famous sites like Dos Ojos, El Pit, and Angelita, with its eerie hydrogen sulfide cloud at 30 m, draw divers from everywhere.

Most cenote diving is cavern diving: guided, within the light zone, on a permanent line, with recreational certification. Going beyond the cavern zone is full cave diving and requires cave training, no exceptions. Buoyancy and fin technique matter enormously here, one careless kick can silt a passage and ruin the famous clarity. Most Yucatan operators run two guided cavern dives a day in small groups of four per guide.