Yeah, I did a feasibility study on underground pumped hydro which you can find here: https://www.terramenthq.com/uphs/

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