[ProtectOurFreedom]

"It just doesn't add up to me."

They are called "pumped storage facilities" and there are many of these around the world. See this Wiki List of Pumped Storage Hydroelectric Power Stations" for those 1,000 MW and larger.

They are used to store cheap power generated at night when demand is low and deliver it to the market during the day when demand is high. They've been used for decades and are quite cheap. An alternative is Compressed Air Energy Storage where you pump high pressure air into old salt mine caverns, but these didn't catch on much.

[Wildbill22]

I live near the Ludington MI facility. At first they discovered it turned aquatic life, fish, into chum until they had to screen it all off.

There has to be a net loss in energy which can only be made up economically by the time of the day that it is used/stored. It is not an answer or a major source of energy. More like a science experiment.