Sistema Ox Bel Ha

The longest surveyed underwater cave on the QRSS list (496,804 m in October 2024), running beneath the Tulum area and the northern edge of Sian Ka'an. CINDAQ, which has been resurveying it since 2018, reported about 524 km at the end of 2024.

Sistema Ox Bel Ha is a vast network of flooded passages under and around Tulum, with QRSS listing about 160 cenote entrances. It has grown both through new exploration and by absorbing caves that were once mapped separately, such as Naranjal in 2011 and Regina in 2020. Divers know parts of it by section names, notably the Naranjal-Mayan Blue area, which is used by cave training courses.

Since 2018 CINDAQ (Centro Investigador del Sistema Acuífero de Quintana Roo) has been resurveying the whole system with modern methods while continuing to push new passage. Its reports say the work started from about 245 km of older data and that more than 185 km of new passage had been found by 2025.

For most visiting cave divers Ox Bel Ha is experienced through a few well-known entrances where permanent guidelines lead into shallow, decorated tunnels with haloclines in deeper sections. Much of the system lies far from roads or on ejido land and is reached only by exploration teams with the landowners' permission.

The system is also a research archive for the aquifer, anchialine ecology and Pleistocene fauna, and CINDAQ has warned that the growth of Tulum, construction of the Tren Maya railway and the new Tulum International Airport put parts of the cave under urban development.

Like other Quintana Roo caves, Ox Bel Ha formed by dissolution of the limestone of the Yucatán Platform along fractures and at the fresh-salt water mixing zone, and was partly dry during glacial low sea levels, when its speleothems formed.

The cave carries fresh groundwater toward the Caribbean coast above a layer of salt water; the halocline between them is deeper farther inland. QRSS lists a maximum surveyed depth of about 57 m in an in-cave pit.

Ox Bel Ha heads the QRSS list of long underwater caves; in 2018 QRSS had briefly ranked Sac Actun ahead of it after the Dos Ojos connection. CINDAQ uses the system as a base for research on freshwater dynamics, anchialine ecosystems, Pleistocene fauna and the wildlife that depends on cenotes as water sources.

Typical cenote water temperatures quoted by operators are about 24-26 °C. Visibility drops where the halocline or silt is disturbed.

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QRSS lists about 160 cenotes (flagged as approximate). CREER describes the system as having several hundred cenote entrances. Mayan Blue, in the Naranjal-Mayan Blue section, is a CREER-designated cave training site; many other entrances are remote or on ejido land.