Geografic locations where it is possible to build pumped storage are limited. Therefore the idea raise to build these underground e.g. in old mines.
An alternative to conventional pumped storage has been developed by erneo. The water is pumped between 2 salt caverns at different depth. The depth difference is used to store energy like in the conventional pumped storage. Such technique has been proposed before however in those proposals the pumps as well as turbines had to be installed below the surface. In the erneo solution salt caverns are pressurized so that the technical equipment can be located above the surface.