Already been done ...
https://www.wbur.org/news/2016/12/02/northfield-mountain-hydroelectric-stationNew England's Largest Battery Is Hidden Inside A Mass. MountainUpdated December 02, 2016
Bruce Gellerman... Today's batteries come in all shapes and sizes. The largest in New England -- and once the world -- was built 45 years ago and is still working.
But it's hidden, on top and deep inside a mountain in north-central Massachusetts.
Northfield Mountain is a naturalist's wonderland. But if you look around, you'll see an unnatural site: a 5-billion-gallon battery.
"So what you're looking at right here is the upper reservoir, and I consider this to be the battery for the station," says Gus Bakas, director of Massachusetts hydro operations for FirstLight Power Resources.
The company owns and operates the Northfield Mountain pumped storage hydroelectric station. The generators are powered by this manmade mountaintop lake.
"So this is our supply, our stored energy, similar to a battery," Bakas continues. "So when we need that battery to turn on, we would take this water and we would run it into the facility to produce power."
The hydroelectric generating facility is located hundreds of feet below the reservoir, hidden deep inside the hollowed-out Northfield Mountain. But you'd never know it, except for an occasional hint. ...
... many, many moons ago.