Milwr Tunnel is part of a mine network (also known as Halkyn District United Mines) nearly 100 km long, the longest in UK and Ireland. Several sections are limestone works and in more than one place have broken into natural cavities. The biggest of these is about 20 m wide, 20 m high and over 50 m long. At one end is a lake about 20 m in diameter. This was used to dump mining debris for several decades without affecting the water level. The lake was tapped and produces as much water as most British river caves. The water contains copper derivatives and fertilises unusual mould. The lake was plumbed and the "bottom" was still not reached at -60 m. Diving (to -20 m) showed a slowly sloping tunnel the diameter of the lake. As the ceiling above the lake is 20 m up, with a further 30 m of natural chambers ascending through a hole in the ceiling, and the lake has been plumbed to 60 m, the depth of natural passage is given as 110 m, despite the fact that the actual depth of the flooded passage is much greater.
The highest access points to the mine system are in the Halkyn area, at around 260 metres each (only one is currently open), and the Sea Level Tunnel exits at 3 metres below high tide, making a through trip of about 260 metres, the deepest artificial through trip in Britain. However, due to collapse causing part of the route to flood even at low tide, the through trip is now considered too dangerous to be undertaken. See Chris Cowdery's site for access information, and Subterranea Britannica for much more information about the mine workings.