The highest canal aqueduct in the world, Grade I listed and a UNESCO World Heritage Site. Completed in 1805 to a design by Thomas Telford and William Jessop.
Apparently the aqueduct is being emptied for cleaning every now and then. There's a plug in the middle which can be seen being pulled by a little boy in this little film on the BBC News website, causing a mighty temporary waterfall in the middle of the gorge.