Two round sinkhole 'eyes' opening into a decorated cavern of stalactites, stalagmites and columns in water so clear it looks…Two round sinkhole 'eyes' opening into a decorated cavern of stalactites, stalagmites and columns in water so clear it looks like air. Snorkellers get the sunbeam shafts; cavern divers get the Bat Cave air dome. Part of Sac Actun, at roughly 375km, the longest underwater cave system on earth. Mayan civilisation was built around cenotes as its only fresh water source, and they were treated as portals to Xibalba, the underworld. Human and animal remains from 12,000+ years ago have been found in these caves, including 'Naia', one of the oldest complete skeletons in the Americas. Divers connected Dos Ojos to Sac Actun in 2018, creating the record system. Anchorage: Inland, shore excursion from the Riviera Maya. Hazards: No site-specific hazards noted, brief normally. More