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Cwmorthin Slate Quarry is a substantial disused underground working found on the shores of Llyn Cwmorthin above the famous slate-mining town of Blaenau Ffestiniog, North Wales. It is one of a number of mines that circle the town, most of which were enormous enterprises in their day. It has a long and complex history beginning in the early 1800's, though being at its most active towards the end of the 1800's. It was last worked (albeit on a very small scale) in the 1980's.

Much of the underground workings are in a good condition and are possible to explore by properly equipped and experienced groups. The mine comprises many miles of tunnel and hundreds of enormous chambers, arranged over six floors (actually, there are more floors than this but generally speaking only six remain open today). Within the workings can be found artefacts ranging from powder horns to timber stairways, from winches to wagons, and from cranes to bridges. Many hours can be spent enjoying this exciting and awe-inspiring environment that will always remain a significant monument to the once world famous Welsh Slate industry.