> You're thinking too small though ;). You can dig a mile deep. And you can lift enormous weights.
But at that point, why wouldn't you just do a classic pumped-storage reservoir?
But at that point, why wouldn't you just do a classic pumped-storage reservoir?
Furthermore, my most optimistic estimates show that underground solid mass gravity storage could compete with pumped hydro on cost anyways. One reason is that solid mass is about 2.5x heavier than water. If excavation is one of your biggest costs, this density is important.