rock to gain access to the slate. The larger mines in the Ffestiniog area include: Llechwedd quarry – now open to the public as a "tourist mine". Most of the underground workings destroyed by quarrying. Llechwedd Slate Caverns Manod – used by the National Gallery, London to store artworks in World War II Maenofferen Oakeley – now partially untopped as an opencast working by Alfred McAlpine plc Cwmorthin Rhosydd Croesor There were also a number of slate mines in the Llangollen area which produced a much darker "black" slate: Berwyn Deeside and Moel Fferna Penarth Another cluster of mines were found in mid Wales centered on Corris. These all worked a pair of slate veins that ran across the Cambrian mountain range from Tywyn in the west through Corris and Aberllefenni in the Dulas Valley to the mines around Dinas Mawddwy in the east. Slate was also mined in Pembrokeshire in places like Maenclochog. Remains Most underground slate mines in north Wales were closed by the 1960s although some open-cast quarries have remained open, including the Penrhyn Quarry and the untopping work at Oakeley in Blaenau Ffestiniog. Work also continues at Berwyn near Llangollen. The final large-scale underground working to close was Maenofferen Quarry (which is owned by the Llechwedd tourist mine) in 1999 although opencast quarrying continues at this location. Many of the mines are now in a state of considerable decay and those that are accessible should not be entered as they are on private property and contain many hidden dangers. Historical and adventurous underground tours are provided at several mines including Rhiwbach (by Go Below), Llechwedd (Zip World and Llechwedd/Quarry Tours Ltd) and Cwmorthin (Go Below). The lower levels of many mines are now flooded and collapses are commonplace; for example, the hillside above the Rhosydd workings has many pits where the roofs of the chambers below have collapsed. Other slate producing areas in Great Britain The most significant non-Welsh British slate industry is that of Cornwall and Devon where the Delabole Quarry is thought to be the largest single quarry in the island. Many of these are no longer worked owing to lower costs of extraction in the larger British workings. The quarrying of slate in Cornwall is known to have been carried out from the late mediaeval period and there was a considerable export trade from some of the quarries near the coasts in the