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.

Today, almost all of the earlier upper workings in both the Back Vein and Old Vein are inaccessible and damaged. It is likely that large tracts of these workings deep in the hill remain in reasonable condition, but are cut off from reach.