Same as a flywheel for storing energy.
These types of energy storage have all kinds of problems.
That the failure mode is very destructive to the surrounding area.

A flywheel loses a bearing, and it will tear up the surrounding concrete bunker.
A concrete block falling from 100m will break a lot of things.

I outlined on STEEM how to build such with falling water.

They are all easy, just a pain to implement. Like, everyone seems to be against them. Especially the electricity selling companies.