Does not appear sustainable overall. Very capital intensive, difficult to service active turbines deep underground. One still needs huge storage underground. Better option may be to have a large storage above ground probably integrating with the pillars supporting the wind mills. Gentle pumping may not be the case if one billion gallons have to be pumped up in eight hours. Also the wind collectors can directly pump water instead of going through generating electricity and pumping.

At one location in Scotland a few years back I heard of a system where they pumped water to a lake in the mountains to act as a storage reservoir for power they generated. I understood that t hey would then run excess water from the lake through a hydrogenerator when they needed the power.

Having read the article, I have some doubts. If one billion litres of water are drained in six hours and (gently) pumped back in eight hours of night winds, what is the purpose of the whole project? The energy required to pump up the water is more than that produced by the water falling to the underground reservoir because of efficiencies. If that much energy is generated by the wind, why not use that directly?

Of course there are losses

The underground scheme satisfies environmental constraints, in ways the much more common above ground [dams] can't.

Wind powered pumps are much cheaper & more reliable than wind powered generators of electricity are.

The idea is to use the turbines to act as peak generators. The peaks usually occur in the day time, allowing the excess capacity in the night time to be stored for future [daytime] use.