Pumped hydro requires a large volume of fluid and a height difference.

Valleys are useful as they provide a large volume that can be capped with relative ease.

An underground reservoir requires a large volume to be excavated (or a pre existing cave system) and a deeper chamber again for the turbines to powered by the fall .. to yet another deeper underground reservoir.

I'm not saying this is impossible, I stating that as an engineering venture (actual civil engineering not software "engineering") there are some real challenges and greater ones than using existing topography.

You're hand waving a lot away with your comment.

To be honest I'm pretty doubtful the economics of excavating a reservoir are going to work. It needs to be very big and very deep, which is going to be very expensive unless you're planning to go all Edward Teller (don't go all Edward Teller).