Large underground quarry working for slab. 6 or 7 underground chambers about 600’ high. Workings extend through hill emerging at pit with abandoned crane. ****

Located in Cwm Hengae, west of the village of Aberllefenni in Gwynedd, the quarry worked exceptionally dense, deep-blue narrow-vein slate. This made it ideal for cutting into large slabs rather than splitting into roofing tiles. The mines contain extensive horizontal tunnels and vast underground chambers, including a huge combined cavern known as Twll Golau. Slate was removed first via the Corris Railway and later by battery-electric trams and lorries.

Aberllefenni Quarry—made up of the Foel Grochan (north side of valley), Hen Gloddfa, and Ceunant Ddu mines (both on south side) —has origins that may reach back to the 14th century, with confirmed activity by 1500. Large-scale commercial quarrying began around 1810, and the site reached its peak workforce in the late 1800s. After a gradual decline through the 20th century, the quarry finally closed in 2003, ending its record as the world’s longest continuously operated slate mine.