Deep inside Elidir Mountain are miles of tunnels and a huge cavern housing one of man`s greatest engineering achievements. Falling water from a lake on top of the mountain turns turbines inside the mountain to generate electricity. The water is then discharged into a lower lake. When there is a low demand for electricity, the water is pumped back to the higher lake, ready to be used again. A visit to Dinorwig Power Station starts at the Visitor Centre, where the technology is explained, then a mini-bus takes the party inside the mountain to see the turbines in action.