Yeah, this is another promising one. The other disadvantage with that design is that the weight of the water is actually working against them. So if their rock/

Yeah, this is another promising one. The other disadvantage with that design is that the weight of the water is actually working against them. So if their rock/concrete is about 2.5x heavier than water, the weight of water is subtracted and you only get 1.5x the weight of water. Also their height is limited by the height of their piston. So I'm not yet convinced that their solution is better than just using underground pumped hydro.