Largest underground quarry outside of Ffestiniog. Workings in 170 chambers on 14 levels, those below 9 now flooded. Long adit leads to chambering which apart for a couple of trial chambers are confined to one vein. An incline leads from the end of the adit to the surface at the West Twll where slate extraction originally commenced. During 1900, the eastern part of the workings collapsed underneath the area of the East Twll and the chambers are loose. The working to the west which are entered from the West Twll are safer and lead to the link passage with Croesor Mine. There are another couple of adits and shafts. The collapsed East Twll has quite a few deep looking crevices amongst the confusion. ***