[1rae]

In Johannesburg we have a whole lot of deep mine shafts that are no longer being used, and one of the main ideas for reusing them is to turn them into hydroelectric power stations. During the night when the electricity is cheaper, the water will be pumped up to the top, and during peak hours the water will be allowed to fall back down and provide power if necessary. It's a similar concept but on a much larger scale.

[PeterisP]

Are they really that big? Stored-hydro stations are efficient because of lake-sized reservoirs, and it takes uncountable miles of tunnels to have the same volume as a medium-sized lake.