Kinda surprised to hear that date, as the UK has a pumped hydro plant[1] which started construction in 1974 and opened in 1984; Tom Scott has a YouTube video about it[2]. It's not all underground, but the machinery is built into a hill and it pumps from a reservoir at the bottom up to the top when electricity is cheap, and is used as a fast-startup generator when demand is higher. Is it really more feasible to build the upper reservoir underground, than on top of, somewhere? Surely "on top of" is higher, easier to get to, easier to flatten, and much cheaper?
The whole problem is there are entire countries that don’t have hills suitable for pumped hydro reservoirs - the Netherlands for example, which has a high energy demand per capita
So the plan is to build an underground reservoir, then dig deeper and build the lower reservoir? At that point, wouldn't it be easier to use the ocean as the upper reservoir, and dig down to build the lower reservoir, and pump seawater around?
How much water do you have to move to power a house, anyway? It must be a lot - a truck pulling a tank of water goes up a hill as part of a journey without worrying about running out of gas, and they could be carrying 40,000 litres or 40 tons of water. Presumably there is no way you could move enough water at home to make a hydro plant - pump it up at night with cheap electricity and run it down at peak time to save money?
I live near Dinorwig pumped storage hydro and can highly recommend the tours they do if you're in the area. The tour bus drives into the mountain and stops literally a few metres from the generating turbines where you can get out and take photos of the huge underground turbine hall.
[1] https://en.wikipedia.org/wiki/Dinorwig_Power_Station
[2] https://www.youtube.com/watch?v=6Jx_bJgIFhI