Rocks have about 4 times the density of water, thus you&#x27;ll need 1&#x2F;4 of the volume of a reservoir to hold as much energy. Since pilling trains is much more expensive than holding water, I can assure you this project will use much more land area than an equivalent lake.<p>Also, this thing requires a straight line high inclination slope and plain areas both up and down. I don&#x27;t know how that compares with high walled valleys, but it is not a very common formation in nature. All said, it may be useful because it uses a different kind of landscape from water storage, so it can increase the total capacity.<p>Anyway, I agree, costs alone will say how much of it is created.