mule has a powerful winch; these are used to take cables in or pay them out to keep the ship centered in the lock while moving it from chamber to chamber. Safety features A failure of the lock gates—for example, caused by a runaway ship hitting a gate—could unleash a flood on the lands downstream of the locks, as the lake above the locks (Gatun Lake or Miraflores Lake) drains through the lock system. Extra precaution against this is provided by doubling the gates at both ends of the upper chamber in each flight of locks; hence, there are always at least two gates in each flight of locks that would have to fail to allow the higher level of water to pass downstream. The additional gates are away from the operating gates. Originally the locks also featured chain barriers, which were stretched across the lock chambers to prevent a ship from running out of control and ramming a gate, and which were lowered into the lock floor to allow the ship to pass. These fender chains featured elaborate braking mechanisms to allow a ship of up to 10,000 tons to be safely stopped. However, given the precise control of ships made possible by the mules, it was very unlikely that these chains would ever be required. With many modern canal users weighing over 60,000 tons, and given the expense of maintaining them, the fender chains were reduced in number in 1976 and finally removed in 1980. Beyond this, the original design of the locks had yet another safety feature, emergency dams that could be swung across the locks at the upper end of every flight. These consisted of swinging bridges, from which girders were lowered to the lock floor; steel shutters could then be run down these girders to block the flow of water. Never used, they were removed in the 1950s. In the late 1930s, the original dams were replaced by new dams, which were raised out of slots in the bottom of the lock chambers, either hydraulically or by compressed air. The new dams were retired in the late 1980s, and today, no emergency dams are in place. Controls Since all the lock equipment is operated electrically, the process of locking a ship up or down can be controlled from a central control room, which is located on the centre wall of the upper flight of locks.