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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, ´óÏó´«Ã½ Radio 5 Live went behind the scenes at Europe’s largest pumped-storage hydroelectric power generator.

Film by Nick Garnett.