In fact I just discovered that there are Pumped-storage hydroelectricity plants (PSPS) because they were planning to build one also to Finland into a old mine (

In fact I just discovered that there are Pumped-storage hydroelectricity plants (PSPS) because they were planning to build one also to Finland into a old mine (with mile long shaft). PSPS could solves the power storage problem for wind and solar as long as there is enough suitable locations to use as PSPS reservoirs.