Sac Actun — "white cave" in Yucatec Maya — is not a single cenote but a vast labyrinth of flooded limestone passages that runs beneath the jungle between Tulum and Akumal. Surveyed at more than 370 kilometres, it is the longest underwater cave system in the world, and many of the more famous cenotes near Tulum — Gran Cenote, Dos Ojos, Casa Cenote, Cenote Calavera — are entrances into it.
You cannot see Sac Actun as a unified site; you visit it through whichever cenote you pick. But its existence is what makes the diving here unique. Certified cave divers train for years before doing the longer traverses; ordinary visitors can snorkel through small, well-lit sections at most of the connected cenotes. The system is also archaeologically significant — divers have recovered Maya offerings and the remains of the oldest known human in the Americas, "Naia," from its passages.