The currents of the Strait flow constantly in both directions—the denser, saltier Mediterranean water sinks to the bottom and is pulled westward into the Atlantic while the waters of the cooler, less salty Atlantic flow steadily eastward along the top, right under the surface. In World War II, 62 German U-boats used the swiftly moving water to their advantage: The submarines would cut their engines upon entering the Strait of Gibraltar from the Atlantic and coast silently on the currents into the Mediterranean, evading detection.