Where is the world’s largest underground river system?

The Yucatán Peninsula in southeastern Mexico contains some of the largest underwater cave and river systems on Earth. The Sac Actun system was recognized as one of the longest known underground river systems, stretching for hundreds of kilometers, and it connects with other extensive flooded passages. These systems formed in porous limestone and are now partially submerged. They are important both scientifically and archaeologically because they preserve ancient landscapes and, in some places, human remains and artifacts.