Sistema Sac Actun

Sistema Sac Actun (Yucatec Maya: sak aktun, lit. 'white cave', Spanish: sistema, lit. 'system') 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. The remains of a mastodon and a human female that might be the oldest evidence of human habitation in the Americas have been found in the cave.

Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).