A hydro electric energy generation structure is disclosed. The structure comprises: a gravity wall forming a closed outer perimeter extending above an upper water level of an existing hydraulic reservoir, and extending below the reservoir floor; at least one water inlet hydraulically connecting a first penstock to a first turbine generator below the water inlet. The structure further comprises: at least one lower water storage reservoir within the perimeter of the gravity wall receiving water from the first turbine generator; at least one pump receiving water from the lower water storage reservoir and pumping it through a pump delivery conduit to at least one upper water storage reservoir above the gravity wall; at least one second penstock delivering water from the upper water storage reservoir to a second turbine generator below; and a tailrace for returning the water into the existing reservoir.