 There are four Francis turbines , which operate as both pumps and generators . These are housed in a cavern within Ben Cruachan , which is 91 @.@ 5 metres ( 300 ft ) long , 23 @.@ 5 metres ( 77 ft ) wide and 38 metres ( 125 ft ) high , with an adjacent transformer hall . The chamber is at a depth of around 300 metres ( 980 ft ) , and is located within a hard granite intrusion . Construction of the power station required the removal of 220 @,@ 000 cubic metres ( 7 @,@ 800 @,@ 000 cu ft ) of rock . Access to the hall is gained by a road tunnel 1 kilometre ( 0 @.@ 62 mi ) long , 4 metres ( 13 ft ) high and 7 metres ( 23 ft ) wide , which is warm and humid enough to allow tropical plants to grow . 
