Milwr Tunnel is part of a mine network (also known as Halkyn District United Mines) nearly 100 km long, the longest traditional passage-based mine network in UK and Ireland (surpassed by modern room-and-pillar mines). 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. A further dive in 2015 showed that the mining debris had reduced the depth of the lake to 42 metres, but since this is artificial and could potentially be removed again, I have kept the original plumbed depth here.
With the lake surface sitting at an altitude of about 9.5 metres (an estimate based on the 1:1000 gradient of the Sea Level Tunnel), the plumbed depth of the lake reaches about 50.5 metres below sea level, the deepest known point of any cave in the UK, but surpassed by some in the Republic of Ireland. With the artificially reduced depth of the lake, the lowest point that can currently be reached is 32.5 metres below sea level, which is the second deepest accessible point of any cave in the UK.
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 (just 1 metre above mean tide), making a through trip of about 259 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.