Sill, the Strait's shallowest point which limits mixing between the cold, less saline Atlantic water and the warm Mediterranean waters. The Mediterranean waters are so much saltier than the Atlantic waters that they sink below the constantly incoming water and form a highly saline (thermohaline, both warm and salty) layer of bottom water. This layer of bottom-water constantly works its way out into the Atlantic as the Mediterranean outflow. On the Atlantic side of the Strait, a density boundary separates the Mediterranean outflow waters from the rest at about depth. These waters flow out and down the continental slope, losing salinity, until they begin to mix and equilibrate more rapidly, much farther out at a depth of about . The Mediterranean outflow water layer can be traced for thousands of kilometres west of the Strait, before completely losing its identity. During the Second World War, German U-boats used the currents to pass into the Mediterranean Sea without detection, by maintaining silence with engines off. From September 1941 to May 1944 Germany managed to send 62 U-boats into the Mediterranean. All these boats had to navigate the British-controlled Strait of Gibraltar where nine U-boats were sunk while attempting passage and 10 more had to break off their run due to damage. No U-boats ever made it back into the Atlantic and all were either sunk in battle or scuttled by their own crews. Internal waves Internal waves (waves at the density boundary layer) are often produced by the Strait. Like traffic merging on a highway, the water flow is constricted in both directions because it must pass over the Camarinal Sill. When large tidal flows enter the Strait and the high tide relaxes, internal waves are generated at the Camarinal Sill and proceed eastwards. Even though the waves may occur down to great depths, occasionally the waves are almost imperceptible at the surface, at other times they can be seen clearly in satellite imagery. These internal waves continue to flow eastward and to refract around coastal features. They can sometimes be traced for as much as , and sometimes create interference patterns with refracted waves. Territorial waters Except for its far eastern end, the Strait lies within the territorial waters of Spain and Morocco. The United Kingdom claims around Gibraltar on the northern side of the Strait, putting part of it inside British territorial waters. As this is less than the