Grid refs : SH 679462
No. of levels : Many
Personal experience? : Yes
Cwmorthin and the adjoining Oakley (Gloddfa Ganol) workings together make up what is reputedly the largest slate mine in the world - certainly it is very large. Gloddfa Ganol is passed on the A470 to the North of Blaenau Ffestiniog; it used to be a tourist attraction, but is now closed. Cwmorthin is above Tan-Y-Grisiau, or on the opposite side of Allt Fawr to Gloddfa Ganol. The two mines join under the hill.
Cwmorthin was being worked until very recently, albeit untopped rather than underground working. The most obvious entrance is a level at the level of Llyn Cwmorthin, called the Lake Level which enters the mine. The overall structure of the mine is extremely complex, however the book Cwmorthin Slate Quarry by Graham Isherwood ISBN 0952297914 contains useful maps.
The underground workings are best entered with somebody who knows where they are going, as any routes are too complicated to describe here.
As mentioned above, Cwmorthin links with Oakley, and a through trip used to be possible. Well, getting into Oakley is possible, exiting is not as untopping work has obliterated all the exits I knew of.
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