I actually think it gets done fairly regularly - its relativly well know in the wales climbing scene too, as a wet weather adventure.
The mine system itself is relativly straight forward. It consists of huge rectangular chambers that all lie parallel to each other and inclined to about 30 degrees.
They are flooded in the bottom - rise up into the blackness above - and the main drag just traverses the chambers from right to left about half way up the chambers.
The entrance adit runs straight into the base of the first chamber.
Right here goes for a description.
Drive up though croesor valley and park as near as you can. Since the entrance is a good 45 minute uphill slog from the car, we got changed up at the entrance, it depends on what your wearing - i was wearing a thin sufing wetsuit under my oversuit.
Entrance is to right of a an old winding house. Long long adit runs straight in for a way until you reach the base of the first chamber and the start of an uphill incline.
climb awkwardly up the incline - past the old pot flue. You pass a couple of windows on the right with over look a flooded chamber until you reach a window on the left over looking a massive chamber with a boulder floor. You will basically now turning left and making your way across the various chambers/bridges in this direction.
The first pitch - there will probably be loads of bits of tat bolts and rope etc. Make your own mind up about the condition of these and weather to use them or not. I reckon 30 meters should be fine.
straight across the chamber is another window - much bigger (more like a col). To the right hand side of the roof supporting pillar is the next pitch - again insitue ropes and stuff - you decide.
Another 20/30 meters should see you ok here. (tip - stand clear of the pitch base once your down - there is loose stuff all over these mines.)
This is where the description changes. The old way on was through a climb down between boulders into a tunnel that marked the start of the railway level that would be followed over the bridges. This is now well and truly blocked.
We descended down to the pool at the base of the chamber - launched our rubber rings and headed for the far right hand side - where a short connecting tunnel leads to the equivalent pool in the next chamber which then has to be crossed back to
the base of the chamber. The water is very cold and deep yet strangely beautiful. Its worth taking the biggest-brightest torch you can lay your hands on - for lighting up some of these chambers.
Once re-assembled on the other side ascend the slope back up the chamber to the railway level - simply follow this to the left (looking up the chamber) along through various enormous chambers and over the several well described bridges.
Bridge 1 - OK
Bidge 2 - No floor - just a tyrolean wire right across.
Bridge 3 - dodgy walk across bendy metal rails - insitue handlines of unknow vintage - to middle support - tyrolean wire rest of way.
On to final lake - again insitu ropes of unknow vintage - 15 meters sees you straight into the water (deep). I pulled on some existing twine which revealed a very nice dingy. so we used that - probably still there.
quick pull on an insitu rope at the yon side gets you back into the continuation passage. Have a laugh at the shrine. and continue onwards. eventually the passage is blocked - so you need to be looking for a passage up to the right. I seem to remember a couple - it was fairly low and there might have been a small cairn at the base of it., this brings you out into rhosydd. again more chambers.
Its a bit of a blur - be we came across a huge chamber open to the light - so we followed that up and out into the base of a small quarry - eventually managed to scramble out of this and then proceeded to get lost in the mist trying to find our way down.
There is the Rhosydd adit if you can find it, that just runs straight in the same direction as the main drag you have been following. (we found the exit on our way back down the mountain. It would be easier to find your way back to the car from there rather that the quarry, but only if you can find the start of it underground. I suspect you just keep going left rather that walking up and out of the huge chamber.
8 hours. Blimey.
If I were you - I'd take rope for the first 2 pitches - which your willing to ditch - (or come back for via the entrace adit). a bit of rope for the bridge crossings that you can take with you - in a leader/ second style. SRT kit.
inflateable device for lake crossings - innertubes/dingy plus twine for retreival. You could take flippers and big ortleibs if your confident enough to just swim the lakes.
Plenty of food. and maybe even a flask - it gets cold in there due to the draft. oh and a huge torch.
Have fun - stay safe - and let us know how you get on - piccies would be great.
