NOVELTY - The storage device (100) has a turbine power unit (130) to generate output electrical power from flow of water into a submersible tank (102). A turbine (136) is operated at initial hydrostatic pressure. A gas vent (148) is attached to tank, provided to enable gas to flow into or out of tank, and provided with a throttling gas valve (162) which is opened and closed automatically to regulate flow of gas into or out of tank to maintain a hydrostatic pressure between a threshold less than initial hydrostatic pressure and a threshold greater than initial hydrostatic pressure.    USE - Underwater pumped-hydro energy storage device for power generation plant.    ADVANTAGE - Since the amount of water in entire ocean or large lake can be remained constant, the storage devices can be supplied with constant water pressure heads. The blade pitch angles of hydro turbine generators can be set to an optimal value to obtain a specific hydrostatic pressure head, to improve efficiency of device. The excessive electricity generated during low demand periods, can be stored in storage device. The power utilities of device can avoid financial, social, and environmental costs associated with building new power generation plant and support infrastructure. The cables can be shortened to bring the tank to a deeper depth when volume of water in water bodies is decreased. Thus, hydrostatic pressure on blades of turbine can be maintained. The tank and turbine can be deployed to different depths for maximum operating efficiency. The tank can be provided with a level sensor that determines whether tank is filled to a predetermined level. Thus, the level sensor can automatically trigger the shut off of valve to prevent inflow water into tank.    DESCRIPTION OF DRAWING(S) - The drawings show the side views of the underwater pumped-hydro energy storage device, and turbine generator of the underwater pumped-hydro energy storage device.    Underwater pumped-hydro energy storage device (100)    Submersible tank (102)    Turbine power unit (130)    Turbine (136)    Gas vent (148)    Throttling gas valve (162)