Sistema Sac Actun is an underwater cave system situated along the Caribbean coast of the Yucatán Peninsula

Yucatán Peninsula

The Yucatán Peninsula, in southeastern Mexico, separates the Caribbean Sea from the Gulf of Mexico, with the northern coastline on the Yucatán Channel...

 with passages to the north and west of the village of Tulum

Tulum, Quintana Roo

Tulum is the largest community in the municipality of Tulum, Quintana Roo, Mexico. It is located on the Caribbean coast of the state, near the site of the archaeological ruins of Tulum...

. Exploration started from Gran Cenote

Cenote

A cenote is a deep natural pit, or sinkhole, characteristic of Mexico and Central America, resulting from the collapse of limestone bedrock that exposes groundwater underneath...

(20.24646°N 87.46403°W) 5 kilometres (3.1 mi) west of Tulum. The whole of the presently explored cave system lies within the Municipality of Tulum (state of Quintana Roo

Quintana Roo

Quintana Roo officially Estado Libre y Soberano de Quintana Roo is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 10 municipalities and its capital city is Chetumal....

).

In early 2007, the underwater cave Sistema Nohoch Nah Chich was connected into and subsumed into Sac Actun making it the longest surveyed underwater cave system in the world for some months. Sac Actun currently measures 215.4 kilometres (133.8 mi) (after connecting Sistema Aktun Hu with 34 kilometres (21.1 mi) in January 2011) and is presently second surpassed by Sistema Ox Bel Ha

Sistema Ox Bel Ha

Ox Bel Ha is a cave system in Quintana Roo, Mexico. It is the longest explored underwater cave in the world and ranking no. 4 including dry caves. As of October 2011 the surveyed length is of underwater passages....

 at 222.3 kilometres (138.1 mi). These two cave systems since early 2007 have been frequently exchanging the title as the longest underwater cave system in the world. Including two connected dry caves (Sistema Yax Muul and Sistema Pierre′s East) makes Sistema Sac Actun with 218 kilometres (135.5 mi) the second longest cave in Mexico and the sixth longest worldwide.

Pleistocene remains

In March 2008, three members of the Projecto Espeleológico de Tulum and Global Underwater Explorers

Global Underwater Explorers

Global Underwater Explorers is a scuba diving organization that provides education within recreational, technical and cave diving. It is a not-for-profit, membership organization, based in High Springs, Florida, United States....

 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 metres (187 ft) the divers located the remains of a mastodon

Mastodon

Mastodons were large tusked mammal species of the extinct genus Mammut which inhabited Asia, Africa, Europe, North America and Central America from the Oligocene through Pleistocene, 33.9 mya to 11,000 years ago. The American mastodon is the most recent and best known species of the group...

 as well as at 43 metres (141.1 ft) a human skull

Human skull

The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...

 that might be the oldest evidence of early man

Human evolution

Human evolution refers to the evolutionary history of the genus Homo, including the emergence of Homo sapiens as a distinct species and as a unique category of hominids and mammals...

in this area to date.