Cwmorthin is a substantial Victorian-era slate mine above the village of Tanygrisau, circling the slate quarrying town of Blaenau Ffestiniog. It has a long and complex history beginning in the early 1800's, with heavy underground development starting around 1860.

It was run by several different companies as a venture in its own right during the late 1800's. The earlier underground workings started at "Lake Level" by the Cwmorthin Slate Company Ltd, ascended upwards in the mountain ultimately for 8 floors in both the Old and Back Vein. Poor working practices and reckless engineering decisions ultimately led to a substantial collapse in 1884 and the end of that company, which failed to recover, in 1888.