Part of the Elan Aqueduct (nprn 95469), itself part of the Elan Valley Water Scheme (nprn 96459), Knighton Tunnel is about 3.9km long and carries the aqueduct under the uplands between Monaughty and Knighton. Its south-western end, at SO 23846 69289, is at the Syphon Outlet of the River Lugg Crossing and at its north-eastern end at SO 27290 71270, it joins the western end of the Gwernaffel Conduit (nprn 412104). A pair of observation towers (nprns 403806 and 412105), presumably used during the construction of the tunnel, stand on Black Hill.
B.A.Malaws, RCAHMW, 16 November 2010.