This involves moving earth in places with suitable topography. The economics depends on both the vertical offset between the two reservoirs, and how much earth has to be moved per unit of reservoir volume.

Algorithms have been created to use GIS data to automatically find potential PHES sites. There are lots of candidates. Australia, for example, has 300x more potential that it would need for a 100% renewable grid.

http://re100.eng.anu.edu.au/global/