[jacquesm]

Elevation translates in to 'head' and that's the single reason that this works well in Switzerland which has something that many other places do not have: very high mountains. With sufficient head the potential energy of a reservoir of modest size can be very impressive, and without you can have a huge reservoir that barely moves the needle.

[skybrian]

I wonder if you could do something similar with underground storage in a mine?

[jacquesm]

Not really enough volume you could only use the lowest levels, and even the largest mines are dwarfed by small lakes. You'd have to put the generators and pumps at the bottom where they'd be easily submerged.

Typically pumped storage uses two lakes, one high and one low. But points for creative thinking :)