Between 1911 and 1920 Alwen Reservoir in Conwy was constructed to provide a water supply to Birkenhead in Wirral (then in Cheshire). It crossed into Cheshire in a 250 yard tunnel beneath the River Dee. The tunnel is syphonic and reaches a depth of around 55 feet.
The access shafts can be seen on both banks - the northern (Cheshire) side being within the Tata Steel Works. These shafts, like the tunnel itself, are lined with cast iron segments. The main tunnel has a diameter of 8 feet and the crown is at least 13 feet beneath the river bed. Construction used compressed air due to the ground conditions beneath the river.