I could see this with a pipe down to deep under water. You pump in air pushing water out the bottom; and let the air out through a turbine to extract power. And

I could see this with a pipe down to deep under water. You pump in air pushing water out the bottom; and let the air out through a turbine to extract power. And, with a big tank or bladder at the end, to keep the pressure nearly constant. Or, deep under the water table, and no tank or bladder needed because there is a big cavern down there. I suppose this could work with a lightweight, immiscible liquid, instead... A pipe down to a closed bladder, and pumping water in, would be simpler.