NOVELTY - An energy storage system (1) has load (4), comprising of e.g. soil, sand, clay, gravel, pebbles, acting on the reservoir (2) below ground level (3) such that displacement of the load away from the starting point increases the volume of the reservoir and the load returns to the starting point upon decrease of the volume. An energy conversion structure (6) increases the reservoir volume under consumption of energy by pumping fluid medium e.g. water, to the volume, and decreases the volume by releasing the fluid medium while converting flow energy in the medium into mechanical energy.    USE - Energy storage system for storing large amounts of energy at fixed location. Can also be used for compensating for fluctuations in demand or production in electrical power supply system and for preventing low-ground area e.g. agricultural area, from being flooded, and used in combination with renewable energy sources e.g. solar, water or wind power.    ADVANTAGE - Energy is stored by use of gravity, eliminate need for hazardous or potentially polluting or costly chemicals. Flow energy is provided by conversion of potential energy stored in load when load is displaced from starting point, allowing energy storage system to be implemented also at flat locations without any natural altitude differences in landscape. Reservoir can be utilized for storage of regular water e.g. salt water or fresh water from lake or sea, or can serve as fresh water supply or emergency reservoir for fire fighting, or can be used for preventing flooding of low grounds in event of emergency. Membrane of reservoir can be made from upper and lower membrane layers of different materials, avoiding descending or ascending of membrane layer in situation where surrounding earth is saturated with water. Layer of sand may be arranged below reservoir to protect membrane against damage. System may be sized for operating water pressure in range between 2 bar and 10 bar, allowing for both use of efficient turbines and standard piping materials. Use of incompressible fluid with density larger than that of air improves efficiency and safety of system, and ensures that reservoir ceiling will move up and down in controlled fashion. Fluid medium leak from reservoir can be sensed using leak detection structure or sensor for acoustic sensing. Two or more reservoirs, each with energy conversion structure, may be employed to increase system flexibility and performance. System may be easily scaled, since it may include amount of standard components to match requirement of specific situation. Additional reservoirs and energy conversion structures may be added without having to deactivate existing elements of system to suit to need for increase in storage capacity. Use of different loads on different reservoirs facilitates efficient use of system as well as more flexible adaptation of specific storage needs and makes use of equally dimensioned energy conversion structures even when reservoirs are placed at different altitudes with respect to energy conversion structures e.g. in sloping landscape. Using enclosed low pressure reservoir prevents fluid medium e.g. water, from evaporating. Reservoir may include lock which allows entrance of service personnel and or robot, facilitating maintenance and inspection.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method of compensating for fluctuations in demand or production in an electrical power supply system; and    (2) a method of preventing a low ground area from being flooded.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of an energy storage system.    Energy storage system (1)    Reservoir (2)    Ground level (3)    Load (4)    Energy conversion structure (6)