There is a German professor for whom the polls are still open on which side of the genius/lunatic line he is. Since the main problem with renewable energy is saving it for the time, when there is no sun or wind, he wants to build a giant energy storage. Available water reservoirs are scarce so his idea is to use the potential energy of a giant plug of granite that would be lifted with water pressure. Here is his website: http://eduard-heindl.de/energy-storage/index-e.html But let me summarize, what I think I understood: - The plug has the same hight as its diameter and would be lifted up to half that hight. Therefore the energy capacity scales as a power of 4! (Volume times hight) - Apparently there are are steel rope cutting techniques employed in South African mines that can be used to cut such a plug in place. - A plug of 500 meters would be enough for Germany's electricity needs for a few days - I don't remember were one would take the water from but it doesn't seem to be a big issue I did some maths on the pressure involved and it seems to pan out. So my question is mostly how would you go about sealing such a plug? What other problems do you see from an engineering point of view? Is is geologically sound?