This image presents a remarkable view of the North Atlantic Ocean, stretching from the equator and western Africa northward to Iceland and Greenland, with sunset occurring over Scandinavia, France, Spain, Algeria, Mali, and the Ivory Coast. As you look at this image, think for a moment about how it was made. Is it a photograph? If so, was it taken from space? Is it a painting, a map, or something else? Clues to interpreting this image can be found in features in the ocean, the clarity of the atmosphere, the appearance of land features in daylight and dark, the geography of the terminator (line of sunset), and the perspective (or field-of-view) of the image. First, look at the ocean—close examination reveals that features on the sea floor are visible, including the continental shelf, and fracture zones associated with the Mid-Atlantic Ridge. Think about your beach experiences: How deep is the water at the beach before you can’t see the bottom any more? Visibility is limited to about a meter (3 feet) on the typical beach, ten meters (33 feet) in the clear waters such as the Caribbean. In fact, only half of one percent of the visible light that hits the surface of the ocean penetrates to 100 meters. Since the average depth of the oceans is 4000 meters (13,000 feet), even the most sensitive camera could not see the sea floor except along the very edge of the coast, and in exceptionally shallow areas. Because light can not penetrate deep ocean water, measurements of bathymetry—the depth of the oceans—are made from sonar mounted on ships and submersibles, and from space by radar altimeters. These data can then be shaded by a computer to show features on the ocean bottom.

