Slate quarry. View of the Penrhyn slate quarry in Bethesda, North Wales. In the foreground is a disused and flooded pit. The currently mined site is behind it (centre left). This quarry is one of the largest in the world. It is well over 200 hectares in size, and goes down over 350 metres. Excavations started at this quarry in 1765. In the distance (top centre), the island of Anglesey can be seen, separated from the mainland by the Menai strait.