In 1976, work started on the ambitious Dinorwic power station in North Wales. One of Europe's largest pumped storage schemes, the project involved construction of a 412m deep vertical shaft, some 3km of tunnels and a massive machine hall deep inside the Elidir Mountain at the foot of the Llanberis Pass.
Opened in 1981, the power station uses water from the upper Marchlyn Mawr reservoir falling more than 500m to drive six turbines in an underground machine hall (153m long by 23.5m wide by 36.5m high) with the tailrace tunnels connecting to the lower Llyn Peris reservoir, 3km to the south west.
At night, water is pumped back to Marchlyn Mawr, when electricity demands are low, to be re-used. Main civil engineering works were undertaken by a consortium headed by Sir Alfred McAlpine.