Many pumped-storage plants just dam an existing valley to create the upper reservoir. Easier than trying to build a gigantic swimming pool on top of a mountain,

Many pumped-storage plants just dam an existing valley to create the upper reservoir. Easier than trying to build a gigantic swimming pool on top of a mountain, and probably safer as well. Of course, in order to do this, you need some very rough terrain, so it's not suitable for flat countries.