The amazing tunnel labyrinth that runs from hills to the sea

Owen Hughes takes a glimpse inside the astonishing Welsh underworld where more than 600 people once worked

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Powell's Lode Cavern off the Milwr Tunnel

UNKNOWN to many, an underground world exists below a swathe of Wales.

The network of mines, lodes, shafts and natural cave systems extends more than 60 miles under the Flintshire and Denbighshire countryside.

Halkyn District United Mines is the longest traditional passage-based mine network in the UK and Ireland.

At the heart of this is the Milwr Tunnel, a mine drainage adit running some 10 miles, from Bagillt, on the Dee Estuary, to Cadole, near Loggerheads, Denbighshire. It cleared the way for further exploration by removing water from the mines.

At its peak, there were 650 people employed in the network of lead, limestone and zinc mines, but numbers dwindled over time and the mines were closed in 1987.

Today, the adit which is managed by United Utilities discharges millions of gallons of water into the Dee Estuary every day. This water had at one time been used by mills sited near the coast.

Entry into the complex is a rarity, with caving clubs able to hold visits. Edward Coghlan, who was last in the network in 2024, shared images from inside.