Gran Cenote, Quintana Roo, Mexico
Collapsed, water-filled caves dot the porous limestone of Mexico's Yucatan Peninsula. The Mayans viewed these cenotes as gateways to the underworld, and the ethereal light of reflected sun makes it easy to believe. The fresh water is so clear that swimming through it feels like flying. Explore around floor-to-ceiling columns that formed inside Gran Cenote when it was a dry cave and venture beneath the overhang toward the darkness, with a clear view to the bottom 36 feet below. Xibalba Dive Center in Tulum provides outings to Gran Cenote and others in the area.