The project – begun in 1974 and costing £425 million[SUP]
[4][/SUP] and taking ten years to complete – was the largest civil engineering contract ever awarded by the UK government at the time. The work was undertaken by an
Alfred McAlpine / Brand /
Zschokke consortium.[SUP]
[5][/SUP] 12,000,000 tonnes (12,000,000 long tons; 13,000,000 short tons) of rock had to be moved from inside the mountain, creating tunnels wide enough for two lorries to pass comfortably, and an enormous cavern 51 metres (167 ft) tall, 180 metres (590 ft) long, and 23 metres (75 ft) wide[SUP]
[6][/SUP] known as "the concert hall". This has also given rise to the station's alternative name of
Electric Mountain. The power station is connected to the National Grid substation at Pentir by 400 kV cables that are buried for approximately 10 kilometres (6 mi), rather than using transmission towers or pylons to transmit the electricity across an area of outstanding natural beauty.