Perhaps best known as the Temple of Doom, Cenote Skeleton is on the list of the 5 most dangerous dive sites in the world. Divers are advised to stay in the sunlit areas as it gets incredibly dark. Many venture out anyway.
Combine that darkness with an intricate network of narrow aisles and divers are in danger of becoming disoriented, lost, and finally running out of air. This exact situation has taken many lives over the years.