About 10-15 years ago my sister's highschool classmate tried to fund a startup to build mechanisms that would store energy by towing buoys down to the bottom of the ocean. I can't find it on the web now, though.

It's an interesting idea compared to other gravity storage solutions because

- It's the volume of the buoy rather than the mass that's important

- It could be colocated with and hooked up to offshore wind farms to smooth their generation

- You don't need to build a super-tall crane or elevated reservoir (but you do have to assemble the thing at the bottom of the ocean which sounds hard)

I haven't heard anything about it in ages so it must have turned out not to be practical.