The gigantic Cwmorthin Slate Quarry first started extraction in around 1810 and went through several ownership changes over its history. It ceased work around 1939 as a separate quarry but post-war was accessed from the neighbouring quarry of Oakeley. At least five underground levels exist, and extensive trackway and haulage machinery was installed to aid extraction. A tramway link to the Ffestiniog Railway was developed and the remains of surface buildings such as slate mills can be seen.
The Friends of Cwmorthin Quarry help maintain safe access through working parties and their website details how to gain access to the site - either as an experienced underground explorer or as part of a group led by a knowledgeable guide.