Does anyone know if something like this has been implemented in some way already? In old mine shafts maybe? The geology has been studied, and some shafts at least there already...
UPHS has never been fully built to my knowledge, but it's been well studied and quite a few projects have tried to get funding for it.
U.S. DOE research from 1984: https://www.osti.gov/biblio/6517343.pdf
Projects trying to work on this: https://utilitymagazine.com.au/pumped-hydro-storage-the-futu...
Other proposed projects: - https://www.waterpowermagazine.com/features/featureinvestiga...
- http://www.eaglecrestenergy.com/project-description.html https://www.osti.gov/biblio/6517343.pdf