The Dinorwig Power Station in Wales – also known, rather cheesily, as the “Electric Mountain”. I took this shot from the same hilltop the Dolbadarn Castle ruins are situated on (see here).
Constructed in the 1970s and opened in 1984, it’s a pump-storage hydroelectric station located deep inside the Elidir Fawr mountain, once the site of the second largest slate quarry in the world.
It’s only used during peak times, to deal with huge and sudden surges in electricity demand. The water stored in the upper Marchlyn Mawr reservoir is discharged through the turbines into the Llyn Peris down below. They then pump the water back up (yup, using electricity!) during periods of low demand (i.e. the middle of the night). I’m pretty sure we studied this place in geography back when I was at school.
Dinorwig is actually open to the public via scheduled guided tours. Unfortunately, they don’t allow photos on the tour, but I can confirm that it’s just like something out of a Bond movie, with huge tunnels hewn out of the rock, its own internal road system and men in hard hats everywhere. The only thing missing was a monorail and a bald dude with a cat.