The Panama Canal locks is a lock system that lifts a ship up 85 feet (26 metres) to the main elevation of the Panama Canal and down again. It has a total of six steps (three up, three down) for a ship's passage. The total length of the lock structures, including the approach walls, is over 1.9 miles (3 km). The locks were one of the greatest engineering works ever to be undertaken when they opened in 1914. No other concrete construction of comparable size was undertaken until the Hoover Dam, in the 1930s.