Like a giant block of Swiss cheese, Mexico's Yucatán Peninsula is riddled with holes called cenotes. Cenotes form when subterranean limestone dissolves, allowing underground water to penetrate. The rock above may cave in, forming a sinkhole that reveals the cool, often crystal-clear water. Other cenotes may remain below the surface, hidden and often unexplored. Cenotes vary in size from very small to several dozen yards across, and recent discoveries have shown that some cenotes lead to a series of underground cave systems that can span several miles in length.