There's lots of exciting progress happening with underground mechanical storage like this. It's so interesting that the U.S. DOE has been studying underground pumped hydro since at least the 1980.(https://www.osti.gov/servlets/purl/6517343)
Today: - Hydrostor just signed a near $1 Billion contract to build their first underground facility in California using advanced compressed air with water to help efficiency. See also: Green Gravity, Gravitricity, Terrament, Renewell, RheEnergise, and more.