Pumped-storage hydro is a known concept. It is the largest existing form of storing peak electricity for time slots with less supply.
Now Fraunhofer has started an experiment with a new idea. According to Georg Küffner writing at FAZ, they built a sphere with concrete, diameter of three meters. Then they immersed that into the Bodensee lake.
The idea is to have a large sphere at the bottom of some lake or ocean and use that as the lower reservoir of pumped storage.
I have no idea if that works out to be cheaper than using mountains. But it might be a good idea for small scale projects sitting right next to some wind park. One advantage that comes to mind: This would probably be fast to build and modular, so you could start out small and easily add capacity as needed. That’s one of the advantages of solar and wind projects as well, so this might be a nice fit for them. In contrast, the old concept needs to build at a large scale fixed for decades.
Anyway, just like the idea of moving mountains, I thought it was worth keeping in mind.