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Sistema Sac Actun (, ) is an underwater cave system situated along the Caribbean coast of the Yucatán Peninsula with passages to the north and west of the city of Tulum. Discovery of a connection to Sistema Dos Ojos in 2018 made it the longest known underwater cave system. as of January 2023, it is the second longest underwater cave system in the world, only surpassed by Sistema Ox Bel Ha.

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