Sistema Sac Actun

A flooded cave network north of Tulum that has absorbed several once-separate systems, including Nohoch Nah Chich (2007), Aktun Hu (2011) and Dos Ojos (2018). QRSS listed it at 379,569 m of underwater passage in December 2024, second only to Ox Bel Ha.

Sistema Sac Actun is a single connected network of flooded passages with well over two hundred cenote entrances on the QRSS list. Under the cave-naming convention used in Quintana Roo, when two surveyed caves are joined the larger name usually carries on, so several systems that divers still know by their own names, such as Nohoch Nah Chich, Aktun Hu and Dos Ojos, are now counted as parts of Sac Actun.

Most divers meet the system through its cenotes on the coastal strip north of Tulum: Dos Ojos, The Pit, Pet Cemetery and Dreamgate are among the entrances whose connection to Sac Actun is documented by QRSS or by operators. Guided cavern tours stay on fixed gold-coloured cavern lines within reach of daylight, while cave-certified teams use the white permanent guidelines that run deeper into the system.

The passages are mostly shallow and richly decorated with stalactites, stalagmites and columns that formed when the caves were dry during glacial low sea levels. Where passages drop below the fresh water, divers pass through a halocline. The system also contains deep sections; QRSS records a maximum depth of about 119 m.

Sac Actun matters well beyond diving. In 2007 its connection with Nohoch Nah Chich made it, briefly, the longest surveyed underwater cave known. The Hoyo Negro pit in the former Aktun Hu section yielded the skeleton known as Naia, dated to 13,000-12,000 years ago, and the INAH-led Gran Acuífero Maya project has described the wider system as a major underwater archaeological site.

QRSS describes the Yucatán caves as dissolution conduits formed in Tertiary limestone along bedding fractures, enlarged where fresh and salt water mix. During glacial periods, when sea level was up to about 125 m lower, the passages drained, ceilings collapsed into the sinkholes now called cenotes, and speleothems grew in the air-filled caves before rising seas flooded them again.

Fresh groundwater flows through the upper passages toward the Caribbean over a near-static layer of salt water. The halocline, the sharp boundary between them, lies deeper the farther a passage is from the coast, and stirring it reduces visibility. CREER describes most Quintana Roo caves as low or no flow, with typical depths of 5-20 m.

Hoyo Negro, a deep pit in the Aktun Hu section, contained a near-complete late Pleistocene human skeleton with extinct fauna; a 2014 study in Science dated it to between 13,000 and 12,000 calendar years ago and linked it to a Beringian-derived mitochondrial lineage. In 2018 INAH's Gran Acuífero Maya project announced the underwater link between Sac Actun and Dos Ojos and reported archaeological contexts in more than 200 cenotes, including remains of extinct Pleistocene animals and Maya ritual material.

Typical cenote water temperatures given by operators are about 24-26 °C. Visibility in fresh water is often very high but drops sharply where the halocline or silt is disturbed.

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QRSS lists 229 cenotes for the system, flagged as approximate. Only some are run as public dive sites; entrances used by operators and documented as part of Sac Actun include Dos Ojos, The Pit, Pet Cemetery, Dreamgate and Nohoch Nah Chich. Hoyo Negro's access path is gated and reserved for controlled scientific diving.