A compressed gas energy storage system comprising: a) an accumulator having an interior configured to contain compressed gas when in use; b) a gas compressor/expander subsystem spaced apart from the accumulator and comprising at least a first compression stage having a gas inlet and a gas outlet in fluid communication with the accumulator interior for conveying compressed gas to the accumulator when in a charging mode and from the accumulator when in a discharging mode; c) a thermal storage subsystem comprising: i. at least a first storage reservoir disposed at least partially under ground and configured to contain a thermal storage liquid at a storage pressure that is greater than atmospheric pressure; ii. a liquid passage having an inlet connectable to a thermal storage liquid source and configured to convey the thermal storage liquid to the first storage reservoir; iii. a first heat exchanger provided in the liquid inlet passage and in fluid communication between the first compression stage and the accumulator, whereby when the compressed gas energy storage system is in the charging mode thermal energy is transferred from a compressed gas stream exiting the gas compressor/expander subsystem to the thermal storage liquid; characterized in that the accumulator has a primary opening, an upper wall, a lower wall and an accumulator interior containing a layer of the compressed gas above a layer of liquid when in use and being at least partially bounded the upper wall and lower wall and further comprising: a) a shaft having a lower end adjacent the primary opening, an upper end spaced apart from the lower end, and a shaft sidewall extending upwardly from the lower end to the upper end and at least partially bounding a shaft interior for containing a quantity of a liquid, the shaft being fluidly connectable to a liquid source/sink via a liquid supply conduit; and b) a partition covering the primary opening and separating the accumulator interior from the shaft interior, the partition having an outer surface in communication with the shaft interior and an opposing inner surface in communication with the accumulator interior; wherein, when in use, at least one of the layer of compressed gas and the layer of liquid bears against and exerts an internal accumulator force on the inner surface of the partition and the quantity of liquid within the shaft bears against and exerts an external counter force on the outer surface of the partition, whereby a net force acting on the partition while the compressed gas energy storage system is in use is a difference between the accumulator force and the counter force and is less than the accumulator force. a) an accumulator having an interior configured to contain compressed gas when in use; b) a gas compressor/expander subsystem spaced apart from the accumulator and comprising at least a first compression stage having a gas inlet and a gas outlet in fluid communication with the accumulator interior for conveying compressed gas to the accumulator when in a charging mode and from the accumulator when in a discharging mode; c) a thermal storage subsystem comprising: i. at least a first storage reservoir disposed at least partially under ground and configured to contain a thermal storage liquid at a storage pressure that is greater than atmospheric pressure; ii. a liquid passage having an inlet connectable to a thermal storage liquid source and configured to convey the thermal storage liquid to the first storage reservoir; iii. a first heat exchanger provided in the liquid inlet passage and in fluid communication between the first compression stage and the accumulator, i. at least a first storage reservoir disposed at least partially under ground and configured to contain a thermal storage liquid at a storage pressure that is greater than atmospheric pressure; ii. a liquid passage having an inlet connectable to a thermal storage liquid source and configured to convey the thermal storage liquid to the first storage reservoir; iii. a first heat exchanger provided in the liquid inlet passage and in fluid communication between the first compression stage and the accumulator, a) a shaft having a lower end adjacent the primary opening, an upper end spaced apart from the lower end, and a shaft sidewall extending upwardly from the lower end to the upper end and at least partially bounding a shaft interior for containing a quantity of a liquid, the shaft being fluidly connectable to a liquid source/sink via a liquid supply conduit; and b) a partition covering the primary opening and separating the accumulator interior from the shaft interior, the partition having an outer surface in communication with the shaft interior and an opposing inner surface in communication with the accumulator interior; wherein, when in use, at least one of the layer of compressed gas and the layer of liquid bears against and exerts an internal accumulator force on the inner surface of the partition and the quantity of liquid within the shaft bears against and exerts an external counter force on the outer surface of the partition, whereby a net force acting on the partition while the compressed gas energy storage system is in use is a difference between the accumulator force and the counter force and is less than the accumulator force.