The Panama Canal locks () are a lock system that lifts ships up to the main elevation of the Panama Canal and down again. The original canal had 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 . 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 original 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, which included a third set of locks, began in September 2007, finished by May 2016 and began commercial operation on 26 June 2016. The new locks allow transit of larger, New Panamax ships, which have a greater cargo capacity than the previous locks were capable of handling. Design There are twelve locks in total. 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. In this usage pattern, the paired locks offer redundancy during maintenance or in the event of mechanical issues. The lock chambers are wide by long, with a usable length of . These dimensions determine the maximum size of ships that can use the canal; this size is known as Panamax. The total lift (the amount by which a ship is raised or lowered) in the three steps of the Gatun locks is ; the lift of the two-step Miraflores locks is . The single-step Pedro Miguel locks have a lift of . The lift at Miraflores actually varies due to the extreme tides on the Pacific side, between at