The power station hidden inside a Welsh mountain
It’s known, locally, as Electric Mountain.
Engie’s hydroelectric station at Dinorwig in North Wales is built deep inside a mountain.
It’s called into service when the National Grid needs a surge in supply and can generate power within 10 seconds.
As part of the Cool Planet season, BBC Radio 5 Live went behind the scenes at Europe’s largest pumped-storage hydroelectric power generator.
Film by Nick Garnett.