The Peterborough lift lock is the highest hydraulic lock in the world. Basically there are two basins (one upper and one lower) which boats sail into. Once the operators deem the basins are full enough they close them up and put the lift into motion. One of the basins goes down while the other comes up. It's really quite an impressive work of engineering. This shot was taken when we were on the upper lock coming down. Here's a link to more information on the lift lock.