I've thought for a while that old open-cut iron ore pits could be prime spots for 'inside-out' pumped hydro, like that system that some German researchers were working which pumped water out of submerged concrete spheres instead of up a mountain.
If you have abandoned deep shafts, they may be smaller in volume than an open pit, but they have several hundred metres of head, so every kilogram of water is much more effective than in schemes with less head.
