Sistema Sac Actun (from Spanish and Yucatec Maya meaning "White Cave System") is an underwater cave system situated along the Caribbean coast of the Yucatán Peninsula with passages to the north and west of the village of Tulum. Discovery of a connection to the Sistema Dos Ojos in 2008 made it the longest known underwater cave system.
The remains of a mastodon and a human female that might be the oldest evidence of human habitation in this area to date have been found in the cave.
Sac Actun measured 230.8 kilometers (143.4 mi) (after connecting Sistema Aktun Hu with 34 kilometers (21 mi) in January 2011) and is as of May 2017 with an explored length of 259.5 kilometers (161.2 mi) only surpassed by Sistema Ox Bel Ha at 270.2 kilometers (167.9 mi). Since early 2007, these two caves frequently exchanged the title of the longest Quintana Roo Speleological Survey underwater cave system in the world. Including connected dry caves makes Sistema Sac Actun with 364.4 kilometers (226.4 mi) the longest cave in Mexico and the second longest worldwide.
In 2018, the discovery of a link between the Sac Actun system (reported to be 263 km long) and the Dos Ojos system in Tulum, Quintana Roo (84 km long) was reported. The connection was found by the Gran Maya Aquifer Project (GAM), led by the cave diver and explorer, Robbie Schmittner. The combined system is reported to be the world's largest underwater cave system known.
In March 2008, three members of the Proyecto Espeleológico de Tulum and Global Underwater Explorers dive team, Alex Alvarez, Franco Attolini, and Alberto Nava, explored a section of Sistema Aktun Hu known as the pit Hoyo Negro. At a depth of 57 meters (187 ft) the divers located the remains of a mastodon, as well as at 43 meters (141 ft) a human skull that might be the oldest evidence of human habitation in this area. Additional bones were located and the skeleton was later identified as that of a teenage female now referred to as Naia.