Sac Actun underwater cave system

Where is Sac Actun?

It runs beneath the jungle between Tulum and the Caribbean coast. Gran Cenote and Dos Ojos are among its entrances.

Why it is important

• Archaeological and fossil finds in its passages
• Part of the region's drinking-water aquifer
• A world-famous cave diving destination

Can you visit?

Swimmers and snorkelers can enjoy entrances like Gran Cenote. Exploring the passages is only for certified cave divers with guides.

Key numbers

The IUGS geoheritage listing gives Sac Actun a length of about 368 km. Research at Northwestern University describes an average depth of about 21 m and a maximum of about 120 m.

Exploration history

Exploration began in 1987 from Gran Cenote. In 2007 it was linked with Sistema Nohoch Nah Chich, and for years it swapped the title of longest underwater cave with Sistema Ox Bel Ha until the Dos Ojos connection in 2018.

Naia and other finds

The Hoyo Negro pit inside the system held Naia, a girl who lived about 12,000–13,000 years ago, along with Ice Age animals.

Protection

Sac Actun is listed among the International Union of Geological Sciences' geoheritage sites (reported). Construction and pollution above the caves, including along the Tren Maya route, are major concerns for divers and scientists.

Cenotes in the Sac Actun system you can visit

• Gran Cenote: swimming and snorkeling near Tulum.
• Dos Ojos: snorkeling and cavern diving.
• The Pit: deep diving in the Dos Ojos park.
• Many other entry cenotes along the Tulum-Coba road (reported).

The 2018 connection

In January 2018 divers of the Gran Acuifero Maya project connected Sac Actun with Dos Ojos, making it the longest known underwater cave system in the world at about 347 km at that time (reported). Since then mapping has continued and the total keeps growing.

What is inside

• Stalactites and stalagmites formed when the caves were dry in the ice age.
• Haloclines where seawater meets fresh water.
• Ice Age bones and Maya artifacts, studied by INAH.
• Blind cave fish and shrimp.

Visiting tips

• Snorkeling needs no certification at Gran Cenote and Dos Ojos.
• Cavern diving needs open-water certification and a guide; full cave diving needs cave training.
• Go early to avoid crowds; carry cash in pesos.
• Never use sunscreen; shower before entering.

How long is Sac Actun?

Published figures put the system at around 370 to 390 km of mapped underwater passages, with more than 220 known cenote entrances. It became the world's longest known underwater cave after it was connected with Sistema Dos Ojos, and it trades that title with Ox Bel Ha to the south.

How it was explored

Divers began mapping it in the 1980s and 1990s. Exploration continues with cave divers laying guidelines through flooded tunnels, some deeper than 100 m.

What has been found inside

Flooded caves preserved bones of Ice Age animals and early humans. The best known is the skeleton nicknamed Naia, found in the Hoyo Negro pit, among the oldest human remains in the Americas.

Why it matters for water

The system carries fresh water to the Caribbean, so pollution from towns and hotels can travel far underground. Protecting cenotes protects the drinking water of the Riviera Maya.