Sistema Sac Actun is an underwater cave system along the Caribbean coast of the Yucatán, running north and west of Tulum. The name is Yucatec Maya, sak aktun, white cave.
The numbers are hard to hold on to. Exploration started from Gran Cenote, five kilometres west of Tulum. In early 2007 the system swallowed Sistema Nohoch Nah Chich and became the longest surveyed underwater cave in the world. It took in Sistema Aktun Hu in January 2011. In 2018 a connection to Sistema Dos Ojos was found. As of January 2023 its explored length is 259.5 km, second in the world only to Sistema Ox Bel Ha at 435.8 km, and the two have swapped the title repeatedly since 2007. Counting the connected dry caves brings Sac Actun to 386 km, the second longest cave in Mexico and third longest anywhere.
The whole explored system lies inside the Municipality of Tulum, in Quintana Roo.
What has come out of the water matters more than the length. In a section of Sistema Aktun Hu known as the Hoyo Negro pit, a dive team from the Proyecto Espeleologico de Tulum and Global Underwater Explorers, Alex Alvarez, Franco Attolini and Alberto Nava, found the remains of a mastodon at 60 metres down. At 43 metres they found a human skull that might be the oldest evidence of human habitation in the Americas. More bones followed, and the skeleton was identified as that of a teenage girl, now known as Naia.
The cenotes are the way in: collapse sinkholes that open the roof to daylight. Some are set up for swimming and snorkelling. The cave proper is for trained cave divers only, and it is unforgiving of anything less.
Mexico
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