200
'No-water' hydropower turns England's hills into green and pleasant batteries
(www.rechargenews.com)
This is a most excellent place for technology news and articles.
What's 108,000 tons in volume?
It's designed to go underground. I see it like water towers, every town has one to get mains water pressure and a store of water.
Wouldn't these be similar for energy? 3 days emergency backup power sounds great, plus it smoothes peaky renewables.
Edit. It's about 40x swimming pools
https://www.themeasureofthings.com/results.php?comp=volume&unit=cm&amt=100000