Carved deep inside the Elidir Fawr mountain in North Wales, the Dinorwig Power Station—often called 'Electric Mountain'—is one of the most extraordinary engineering feats of the 20th century. Dinorwig is a pumped-storage hydroelectric scheme designed to solve the problem of grid energy storage at a massive scale. The facility consists of two lakes: Marchlyn Mawr at the top of the mountain and Llyn Peris at the bottom.
Connecting them is a vast network of sixteen kilometers of underground tunnels carved through slate rock. During periods of low electricity demand, cheap off-peak power pumps millions of cubic meters of water up to the top reservoir. When national power demand suddenly spikes—such as during the famous British 'TV pickup' when millions of electric kettles are turned on simultaneously—the control room opens massive spherical valves.
High-pressure hydraulic head forces water down steep penstock shafts, surging into six reversible Francis pump-turbines located in a cavernous underground chamber large enough to fit a cathedral. The immense hydraulic pressure drives the turbines, accelerating the plant from a standing start to full 1,800-megawatt output in under 16 seconds. Because the entire power house is buried beneath hundreds of meters of solid rock, it can safely contain the extreme hydrostatic pressures generated without disrupting the natural surface landscape of the national park.