Strait of Gibraltar : This image is a mosaic of two photographs taken June 3, 2004 with a Kodak DCS760 digital camera equipped with a 180 mm lens by astronauts aboard the International Space Station viewing large internal waves in the Strait of Gibraltar. These subsurface internal waves occur at depths of about 100 m, but appear in the sunglint as giant swells flowing eastward into the Mediterranean Sea. The narrow Strait of Gibraltar is the gatekeeper for water exchange between the Atlantic Ocean and Mediterranean Sea: a top layer of warm, relatively fresh water from the Atlantic Ocean flows eastward into the Mediterranean Sea and in return, a lower, colder, saltier layer of water flows westward into the North Atlantic ocean. A density boundary separates the layers at about 100 m depth. The water flow is constricted in both directions because it must pass over a shallow submarine barrier, the Camarinal Sill. When large tidal flows enter the Strait, internal waves (waves at the density boundary layer) are set off at the Camarinal Sill as the high tide relaxes. The waves - sometimes with heights up to 100 m - travel eastward. Even though the waves occur at great depth and the height of the waves at the surface is almost nothing, they can be traced in the sunglint because they concentrate the biological films on the water surface, creating slight differences in roughness.