Ocean and lake vessels are raised and lowered between the Atlantic Ocean and Lake Superior by locks such as Eisenhower Lock. A ship enters the chamber while the gate is closed, water enters or empties by gravity through valves until the chamber reaches the upstream or downstream level, attendants open and close gates and tie off the ship, and the vessel then departs. Navigators must guide massive ships into a space only slightly larger than the ship's beam. The lock has a filling time of 7 minutes, an emptying time of 7 minutes, a total time in the lock of 20 minutes, uses 22 million gallons of water, and raises or lowers ships 42 feet. Ships from 50 different countries pass through carrying goods around the globe as the St. Lawrence Seaway links interior U.S. and Canadian markets with the world.