Inside the Scottish mountain which is home to a power station
Scottish Power is planning to double the size of its hydro-electric power plant which creates and stores energy.
Cruachan power station pumps water 400 metres uphill at night, when energy from wind farms is cheap, and stores it in a mountain-top reservoir.
The water is released downhill during the day, to generate electricity when demand peaks and power prices are expensive.
The firm says it will help the UK create energy without polluting fossil fuels.