The Peterborough Liftlock, built in 1904 is the highest hydraulic liftlock in the world. Part of the Trent Severn Waterway it is known as Lock 21. It’s dual lifts raise and lower boats 20m. It was designated a national historic site in 1979. A feat of engineering the lock requires no additional power to operate using gravity and counterweight to simultaneously raise and lower it’s two lifts. In the winter the canal at the foot of the lock has its water level lowered and in maintained as free public skating.