Let's say you're just wandering around the Yucatán Peninsula, looking for Mayan ruins or training for a marathon or whatever. Then all of a sudden the ground gives way under you, your leg slips, and next thing you know you're waist deep in clear water at the bottom of an underground cave. Talk about one hell of a discovery.
And that's almost literally how many of the nearly 7,000 cenotes in Mexico's Yucatán Peninsula were discovered. These secret underground caverns are formed when limestone caves in, leaving a hole in the earth that leads to a pool of often gorgeous turquoise-colored water. Some cenotes (pronounced cey-NO-tays and derived from a Mayan word for "well") are part of a massive underground network of rivers and caves that remain, for the large part, unexplored. Others are just afternoon swimming holes or cave-diving destinations.