The scheme was constructed in the abandoned Dinorwic slate quarry. To preserve the natural beauty of Snowdonia National Park, the power station itself is located deep inside the mountain Elidir Fawr, inside tunnels and caverns. The project - begun in 1974 and taking ten years to complete at a cost of £425 million - was the largest civil engineering contract ever awarded by the UK government at the time. The work was undertaken by an Alfred McAlpine / Brand / Zschokke consortium. Twelve million tonnes (12,000,000 long tons; 13,000,000 short tons) of rock had to be moved from inside the mountain, creating tunnels wide enough for two lorries to pass comfortably and an enormous cavern tall, long, and wide known as the concert hall. The power station comprises of tunnels, one million tons of concrete, 200,000 tons of cement and 4,500 tons of steel. The scheme paid for itself within two years.