A power take-off and storage system comprising: - a power take-off system (50, 70); and - an underwater energy storage system for storing energy beneath the surface of a waterbody, wherein the underwater energy storage system comprises a reservoir structure (5; 105), which comprises a base structure by which the reservoir structure is resting at the bottom of the waterbody, and wherein the reservoir structure comprises a pressurizing reservoir (7A, 7B) and a depressurizing reservoir (8A, 8B), each of which is being held relative to said base structure, and each of which is holding a working liquid separated from the water of the waterbody, and wherein the pressurizing reservoir (7A, 7B) is holding the working liquid beneath the surface of the waterbody, and wherein the pressurizing reservoir (7A, 7B) has a deformable wall structure (17A, 17B) and is arranged and configured for pressurizing the working liquid contained within the pressurizing reservoir by deformation of the deformable wall structure under influence of the hydrostatic pressure of the water acting on the deformable wall structure, and wherein the depressurizing reservoir (8A, 8B) has a rigid wall structure (6, 14, 18A, 18B) arranged and configured for shielding the working liquid within the depressurizing reservoir from pressurization under influence of the hydrostatic pressure of the water, and wherein the underwater energy storage system further comprises an energy storing subsystem (11A, 11B) configured for storing energy by means of increasing the potential energy of the working liquid within the reservoir structure by displacing part of the working liquid from the depressurizing reservoir into the pressurizing reservoir, as well as an energy retrieving subsystem (12A, 12B) configured for retrieving stored energy by means of decreasing the potential energy of the working liquid within the reservoir structure by releasing part of the working liquid to flow from the pressurizing reservoir into the depressurizing reservoir; characterized in that: - the depressurizing reservoir (8A, 8B) is holding the working liquid beneath the surface of the waterbody; and - the power take-off system is resting at the bottom of the waterbody by the base structure of the reservoir structure of the underwater energy storage system. - a power take-off system (50, 70); and - an underwater energy storage system for storing energy beneath the surface of a waterbody, wherein the underwater energy storage system comprises a reservoir structure (5; 105), which comprises a base structure by which the reservoir structure is resting at the bottom of the waterbody, and wherein the reservoir structure comprises a pressurizing reservoir (7A, 7B) and a depressurizing reservoir (8A, 8B), each of which is being held relative to said base structure, and each of which is holding a working liquid separated from the water of the waterbody, and wherein the pressurizing reservoir (7A, 7B) is holding the working liquid beneath the surface of the waterbody, and wherein the pressurizing reservoir (7A, 7B) has a deformable wall structure (17A, 17B) and is arranged and configured for pressurizing the working liquid contained within the pressurizing reservoir by deformation of the deformable wall structure under influence of the hydrostatic pressure of the water acting on the deformable wall structure, and wherein the depressurizing reservoir (8A, 8B) has a rigid wall structure (6, 14, 18A, 18B) arranged and configured for shielding the working liquid within the depressurizing reservoir from pressurization under influence of the hydrostatic pressure of the water, and wherein the underwater energy storage system further comprises an energy storing subsystem (11A, 11B) configured for storing energy by means of increasing the potential energy of the working liquid within the reservoir structure by displacing part of the working liquid from the depressurizing reservoir into the pressurizing reservoir, as well as an energy retrieving subsystem (12A, 12B) configured for retrieving stored energy by means of decreasing the potential energy of the working liquid within the reservoir structure by releasing part of the working liquid to flow from the pressurizing reservoir into the depressurizing reservoir; - the depressurizing reservoir (8A, 8B) is holding the working liquid beneath the surface of the waterbody; and - the power take-off system is resting at the bottom of the waterbody by the base structure of the reservoir structure of the underwater energy storage system.