How do they accomplish that? There is only so much potential energy in water at elevation X+100 ft. That energy is dissipated descending to elevation X and must again be expended to pump the water back up again. Since generators are inefficient, it always takes more to pump back up than is generated coming down. Otherwise it would be a perpetual motion machine.
Turn a turbine generator on it's side and then run a hundred of them the same way down the length of the shaft. Then do the same for the other three sides of the shaft.
Start pouring water down the sides of the shaft and gravity does the rest. It doesn't take as much energy to pump the water back up as it generates going down because instead of passing just one turbine the same water passes hundreds of them generating enough to pump the water back up and they sell off the rest. Keep in mind these are not massive turbines that you'd see in a dam.
It's not perpetual motion because you do have to use energy to run the pump. Without the pump the water stops and so does the power. On top of it there is routine maintenance that needs to be done.
From a pure energy standpoint you are correct. However, from an economic viewpoint it's a little different. For large commercial users of electricity, charges for electricty are more expensive during the day (when there is high demand) than at night, when it's cheap.
There are a number of buildings in Chicago that are air conditioned with ice. Rather than run air conditioners during the day, when everyone is running their air conditioners, they blow air across ice and then through the buildings. At night, they run the refrigeration compressors (with less expensive electricity) to freeze the resultant water back into ice. The scenario proposed to you may run in a similar fashion; pump the water back up using cheap electricity, generate electricity during the day when it's more expensive.
I read about a variant to the falling water mine generating technique. That was to pump water up to a large pond or holding tanking on a hill/mountain. Pump the water up during the night when power demands are low, let it flow and generate during the day when the grid needs more energy.