|Date||24 September 2011|
|Location||Dinorwic Quarry, Llanberis||SH 59610 60519; 53.12329°N, 4.09943°W|
The south-western aspect of Elidir Fawr, above the lake Llyn Peris and the town of Llanberis, is dominated by the sculpted landscape formed by the now quiet galleries of Dinorwic Quarry. Commercial quarrying of slate started there in 1787 and production continued until the business closed in 1969.
The quarry was originally part of the Vaynol Estate, owned by the Assheton Smith family. It was once the second largest slate quarry in the world, after the nearby Penrhyn Quarry. At its peak in the late 19th century, it employed 3,000 quarrymen and produced 100,000 tonnes of finished slate a year. Slates were exported from the quay at Port Dinorwic (Y Felinheli) and a tramway from the quarry to the port was constructed in 1824. This was superseded in 1842 by the Padarn Railway.
When the quarry closed, its workshops at Gilfach Ddu were acquired by the National Museum of Wales and are now home to the National Slate Museum. The quarry itself is now owned by First Hydro, operators of Dinorwig Power Station. This pumped-storage hydroelectricity plant uses Llyn Peris as its lower reservoir and Marchlyn Mawr as its upper one. The power station was completed in 1984, having taken 10 years to construct. It is housed deep inside Elidir mountain itself and its Machine Hall is the largest man-made cavern in Europe. There are 16 km of underground tunnels and the total volume of rock excavated from underground amounted to 1 million cubic metres (3 million tonnes).