Where It Is: The limestone bedrock that underlies Mexico’s Yucatán Peninsula is pocked with freshwater-filled sinkholes called cenotes. For divers, one of the best known is the 48-foot-deep Cenote Taj Maja, just south of the Caribbean coastal town of Playa del Carmen.
What to Look for: In the still and crystal-clear waters of Cenote Taj Maja, a 60-foot-wide sinkhole leads to an immense network of underwater caverns. Divers descend and swim through rock passageways to cathedrals of stalagmites, stalactites, and coral fossils. Deep down, seeping seawater from the nearby ocean creates a strange layered, mirrorlike phenomenon known as the halocline effect, where a layer of saltwater meets freshwater above.