Comment by squirtle24

Thinking out loud here, but why sand and not water? Yes, water is only 1000kg/m^3, but probably much easier to transfer around using pumps and pipes. Water can make good use of the horizontal space within the mine, unlike sand. (The graphic in the article shows them using conveyor belts or trucks to move the sand horizontally which seems silly.) Thus the mine could basically be a mini pumped hydro power station, build a new reservoir at the top and use the mine as the bottom reservoir, then pump water water between the two.