Gatun Lock in the Panama Canal December 1999 before United States returned sovereignty to Panama. 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. 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.  There are two independent transit lanes, since each lock is built double. The size of the locks limits the maximum size of ships that can transit the canal; this size is known as Panamax.  Construction on the Panama Canal expansion project began in September 2007. This will double the canal's capacity, allowing more and larger ships to use the canal. The project is expected to be completed in April 2015. The canal has three sets of locks. A two-step flight at Miraflores, and a single flight at Pedro Miguel, lift ships from the Pacific up to Gatun Lake; then a triple flight at Gatun lowers them to the Atlantic side. All three sets of locks are paired; that is, there are two parallel flights of locks at each of the three lock sites. This, in principle, allows ships to pass in opposite directions simultaneously; however, large ships cannot cross safely at speed in the Culebra Cut, so in practice ships pass in one direction for a time, then in the other, using both "lanes" of the locks in one direction at a time.