I think putting tanks in the bottom of abandoned mineshafts, and filling them through positive displacement pump/motors is the way to go. You get energy as the water goes down, and use it to pump the water back up. The costs are the mine, and the maintenance required to keep the lift working, and the whole thing from collapsing... and two tanks / pools.. one at the bottom, one at the top, piping between the two, and a positive displacement pump, with a motor/generator.

All known technologies.