The Peterborough Lift Lock is a boat lift located on the Trent Canal in the city of Peterborough, Ontario, Canada, and is Lock 21 on the Trent-Severn Waterway. For many years, the lock's dual lifts were the highest hydraulic boat lifts in the world, raising boats . This was a considerable accomplishment in the first years of the 20th century, when conventional locks usually only had a rise. In the 1980s, a visitor centre was built beside the lock. It offers interactive simulations of going over the lift lock in a boat, and historical exhibits detailing the construction of the lift lock. Residents and visitors skate on the canal below the lift lock in the winter. The Peterborough Lift Lock was designated a National Historic Site in 1979, and was named an Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers in 1987. The Trent-Severn has a similar hydraulic lift lock, the Kirkfield Lift Lock, at its summit near Kirkfield, with basins of the same dimensions, but which has a smaller vertical lift. Description and operation The lock has two identical bathtub-like ship caissons in which vessels ascend and descend. Both caissons are enclosed at each end by pivoting gates, and there are pivoting gates at the upper and lower reaches of the canal at the junctions with the caissons. The gates on the caissons fit into slots on the gates on the reaches, so that they open in unison. Each caisson sits on a diameter ram, the shafts for which are sunk into the ground, are filled with water, and are connected with a pipe that has a crossover control valve. The caissons are guided up and down on either side by rails affixed to concrete towers. The caissons are long, wide, and deep and when filled with of water weigh . No external power is needed: the lift lock functions by gravity alone using the counterweight principle. One caisson always ascends and the other always descends during each locking cycle. When one caisson reaches the top position, it stops below the water level of the upper reach, and the control valve is closed; Siemens ultrasonic sensors are used to help determine the differential. The upper reach and top caisson gates open, and water flows into the top caisson until the level equalizes. The weight of the extra foot of water is , making the total weight