An astronaut aboard the International Space Station took this photograph of small island cays in the Bahamas and the prominent tidal channels cutting between them. For astronauts, this is one of the most recognizable points on the planet. The string of cays, stretching 14.24 kilometers (8.9 miles) in this image, extends west from Great Exuma Island (just outside the image to the right). Small tidal changes on the banks cause great quantities of water to flow daily through the narrow channels between the cays, first in one direction and then the other. The darker blue sections are the deepest parts of the channels, where the water flow has cut through the rock ridge that makes the line of cays. The surrounding water is shallow (less than 25 meters, or 80 feet) and appears light blue.