[hannob]

Pumped water only works if you have the right conditions. I.e. you need a certain altitude difference and enough space to have an upper and a lower water storage facility.

Pumped water storage is very efficient and an established technology, so when it's possible it's a good choice, but it's not possible everywhere.

[Murkin]

From what I gather in their explanation, they use air pressure to push water out of an underground container.

The energy is reclaimed when the pressure is released (i.e. water can fall back and fill the container).

How is the energy density different that what is provided by the water's static energy due to gravity?