In some places of Mexico's Yucatan peninsula, the forest floor has fallen in, leaving holes in the ground called cenotes. Cenotes were considered openings to the underworld by the ancient Mayans. In this video, Jonathan teams up with an expert cave diver to explore these spooky underground rivers and finds some amazing geologic formations. The accompanying lesson looks at the density and salinity of water that interact to form a halocline, and offers an activity to float an object on a halocline.