oh I've heard about that one too actually - don't they suspend a large weight and use its potential energy as storage?

I'm a sucker for these back-to-basics engineering projects. High tech batteries be damned, let's just use potential energy for storage!

There is a German company suggesting to cut a large piece of rock (think: hundreds of meters in diameter and depth) lose and lift that by pumping water beneath it. A moving mountain, it would be. Totally crazy stuff, I should check if that ever made any progress.

Here is a German article about it. As far as I remember, the real issue is that pumps/generators with the required powers are very expensive. https://www.ingenieur.de/technik/fachbereiche/energie/lageen...

Omega Tau Podcast had a great episode about this company and project:

https://omegataupodcast.net/299-gravity-storage/

I did not know that yet, thanks for pointing it out!