Sistema Sac Actun

Sistema Sac Actun is a flooded cave system beneath the limestone bedrock of the Yucatan Peninsula near Tulum, Mexico, and, at well over 300 kilometers of surveyed passage, is recognized as the longest known underwater cave system in the world. Like the peninsula's other cenotes, it formed through the dissolution of porous limestone by slightly acidic groundwater over a very long period, carving out an extensive network of flooded galleries and chambers that divers have progressively connected and mapped over decades of exploration; the system was found to connect physically to the nearby Dos Ojos cave system, previously considered separate, becoming one combined network under the Sac Actun name. Its water is famously, almost perfectly transparent, filtered clean by its passage through the limestone, and portions of the system preserve submerged stalactites and stalagmites formed during past ice ages when sea levels were far lower and the passages were dry air-filled caves rather than flooded ones. Archaeologists have also recovered ancient human and Ice Age megafauna remains from its passages, preserved by the enclosed, oxygen-poor underwater environment. This entity documents Sistema Sac Actun specifically as a geological formation, the longest known underwater cave system in the world, carved by groundwater dissolution through Yucatan limestone.