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. It was originally built to drain the lead mines beneath Halkyn Mountain, which were plagued with flooding in their lower levels, but enabled the exploitation of new lodes and was variously used for the extraction of lead, zinc and limestone during its working history. It is part of a network of mines, lodes and natural cave systems – the Halkyn United Mines – that extends for up to 100 kilometres, the longest in the United Kingdom.