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

Yeah, I did a feasibility study on underground pumped hydro which you can find here: https://www.terramenthq.com/uphs/ 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 https://www.osti.gov/biblio/6517343.pdf Projects trying to work on this: https://utilitymagazine.com.au/pumped-hydro-storage-the-future-for-old-mines/ https://utilitymagazine.com.au/pumped-hydro-storage-the-futu... Other proposed projects: - https://www.waterpowermagazine.com/features/featureinvestigating-aquabank- https://www.waterpowermagazine.com/features/featureinvestiga... - http://www.eaglecrestenergy.com/project-description.html http://www.eaglecrestenergy.com/project-description.html https://www.osti.gov/biblio/6517343.pdf https://www.osti.gov/biblio/6517343.pdf