This is already being done, by pumping water uphill: http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity http://en.wikipedia.org/wiki/Pumped-storage_hy

This is already being done, by pumping water uphill: http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity I'm sure there are a bunch of interesting tradeoffs between using water or a bunch of rocks though.