Sistema Sac Actun, 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. The entire portion of the explored cave system resides in the Municipality of Tulum.
In early 2007, Sac Actun became the longest explored underwater cave system for a few months, after the underwater cave Sistema Nohoch Nah Chich was absurd into it. Sac Actun is 230.8 mi (230.8 km) and is surpassed only by Sistema Ox Bel Ha, which is 159.5 mi (256.7 km). These two cave system consistently compete to be the longest cave system in the world. If connected dry caves and Sistema Dos Ojos are included, Sistema Sac Actun becomes the longest cave in Mexico and the second longest worldwide.
In March 2008, in the pit Hoyo Negro, remains of a mastodon, as well as the human skull of what might be the earliest man in that area to date, were found by theee members of the Proyecto Espeleológico de Tulum and Global Underwater Explorers dive team, Alex Alvarez, Franco Attolini, and Alberto Nava.