Abstract: Energy storage system comprising: a first tunnel shaft extending in an upright direction from a ground level to a predetermined underground level; an underground chamber at the predetermined underground level in the first tunnel shaft; a water reservoir is provided at the ground level a second tunnel shaft extending in a lying direction at the predetermined underground level, the second tunnel shaft forming a second water reservoir; at least one pipe extending through the first tunnel shaft interconnecting the first water reservoir and the second water reservoir for enabling water to flow between the ground level and the predetermined underground level; and at least one electrical pump and at least one electrical turbine operationally connected to the at least one pipe to enable a controlled charging and discharging of the energy storage system by running an upward and a downward flow of water via the pumps and the turbines, respectively.