A compressed gas energy storage system comprising: a) an accumulator having a primary opening, an upper wall, a lower wall and an accumulator interior at least partially bounded the upper wall and lower wall, the accumulator for containing a layer of compressed gas atop a layer of liquid when in use; b) a gas compressor/expander subsystem spaced apart from the accumulator; c) a gas conduit having an upper end in communication with the gas compressor/expander subsystem and a lower end in communication with accumulator interior for conveying compressed gas into the compressed gas layer; d) 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 containing a quantity of a liquid, the shaft being fluidly connectable to a liquid source/sink via a liquid supply conduit; e) a liquid conduit providing fluid communication between the liquid in the shaft interior and the layer of liquid in the accumulator, whereby liquid can flow between the shaft interior and the layer of liquid in the accumulator in response to changes in the pressure of the layer of compressed gas; f) a thermal storage subsystem provided in fluid communication between the gas compressor/expander subsystem and the accumulator, the thermal storage apparatus including a sensible storage apparatus using a sensible storage fluid comprising water whereby thermal energy is extracted from the compressed gas exiting the gas compressor/expander subsystem at an exit temperature and stored in the thermal storage subsystem such that the temperature of the gas exiting the thermal storage subsystem is reduced to a storage temperature that is less than the exit temperature, and the water in the sensible storage apparatus is heated to a storage temperature of 100 degrees Celsius or higher and is maintained at an operating pressure that is sufficient to generally keep the water in its liquid phase during use; g) a partition positioned at the lower end of the shaft and 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 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 a primary opening, an upper wall, a lower wall and an accumulator interior at least partially bounded the upper wall and lower wall, the accumulator for containing a layer of compressed gas atop a layer of liquid when in use; b) a gas compressor/expander subsystem spaced apart from the accumulator; c) a gas conduit having an upper end in communication with the gas compressor/expander subsystem and a lower end in communication with accumulator interior for conveying compressed gas into the compressed gas layer; d) 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 containing a quantity of a liquid, the shaft being fluidly connectable to a liquid source/sink via a liquid supply conduit; e) a liquid conduit providing fluid communication between the liquid in the shaft interior and the layer of liquid in the accumulator, whereby liquid can flow between the shaft interior and the layer of liquid in the accumulator in response to changes in the pressure of the layer of compressed gas; f) a thermal storage subsystem provided in fluid communication between the gas compressor/expander subsystem and the accumulator, the thermal storage apparatus including a sensible storage apparatus using a sensible storage fluid comprising water whereby thermal energy is extracted from the compressed gas exiting the gas compressor/expander subsystem at an exit temperature and stored in the thermal storage subsystem such that the temperature of the gas exiting the thermal storage subsystem is reduced to a storage temperature that is less than the exit temperature, and the water in the sensible storage apparatus is heated to a storage temperature of 100 degrees Celsius or higher and is maintained at an operating pressure that is sufficient to generally keep the water in its liquid phase during use; g) a partition positioned at the lower end of the shaft and 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;