When it was fully commissioned in 1984, Dinorwig Power Station was regarded as one of the world’s most imaginative engineering and environmental project. Dinorwig is comprised of 16km of underground tunnels, deep below Elidir mountain. Its construction required 1 million tonnes of concrete, 200,000 tonnes of cement and 4,500 tonnes of steel. Dinorwig’s reversible pump/turbines are capable of reaching maximum generation in less than 16 seconds. Using off-peak electricity the six units are reversed as pumps to transport water from the lower reservoir, back to Marchlyn Mawr.
[youtube https://www.youtube.com/watch?v=SDwKnoW5AZw&rel=0]
[youtube https://www.youtube.com/watch?v=SYQUkdL-2iU&rel=0]
— source electricmountain.co.uk