Cenote Azul (the Blue Cenote) in Yucatan, Mexico is separated from Laguna Bacalar by no more than 100 yards or so. Azul is the deepest cenote in Mexico, possibly in the world. In its deepest part it's 290 ft plus and at its widest point it's 600 ft across. Cenotes are underground caverns where the roof has collapsed and the cave subsequently filled with water. The whole of the Northern part of the Yucatan Peninsula is one huge limestones slab. From coast to coast between the Gulf Of Mexico and the Caribbean there are hundreds maybe even thousands of centoes of varying size and depth, Most are linked by a vast network of underground rivers and the coastal ones are linked to the sea. Because of the intricate rock formations below, usually crystal clear water and often great numbers of colourful fish they are wonderful for snorkelling and diving too, though on my part I have only snorkelled.