Comment ocean-based storage could be possible (Score 1) 45
It does not even have to be mountains, or a location at land. You could for example construct a dike at sea to create a polder (as done in the Netherlands) and use the potential of different water levels inside and outside the dike for energy storage. Cost of dike building should be roughly approximate to circumference of the polder, while energy storage potential is proportional to area of the polder - thus a technology that scales well with large storage requirements.
Or you could possibly use the water pressure at depth, without having a structure going all the way to the surface, as in this project: https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fwww.iee.fraunhofer.de%2F...