The Milwr Tunnel is a mine drainage adit running some 10 miles from the hamlet of Cadole near Loggerheads, Denbighshire to Bagillt on the Dee Estuary in North Wales.
The mines involved hoped that the construction of the tunnel, by allowing them to work rich seams of ore at depth without incurring high pumping costs, would enable them to meet their low-cost international competition head-on.
[2] Tunnelling was commenced in July 1897 at Boot End, Bagillt, from a point 9 feet below high water mark on the Dee foreshore, where self-acting flood doors were fitted.
In 1908 driving stopped 2 miles from the portal at Caeau Mine, the limit of the company's mineral rights, at which time the tunnel was draining some 1.7 million gallons of water per day through the drainage channel cut in its floor.
Low lead prices in 1938 caused the majority of the workforce to be temporarily laid off, but Pilkington's Glass began using the tunnel to excavate high-quality limestone from 1939; this quarrying continued until 1969, creating large artificial caverns west of Olwyn Goch.
Although a complete trip through to the portal is no longer possible due to roof falls and consequent flooding in the lower sections, occasional access is permitted to cavers and mine explorers, who have mapped many of the old workings.