Abstract: A method for underground storage of a gas is disclosed in which the stored gas or top gas is physically separated from the cushion gas to avoid undesired intermixing of the two gases. This enables a less expensive cushion gas to be used resulting in a significant cost savings in the development and use of an underground storage cavern. The gas separation can be accomplished by dividing the cavern into two portions using a flexible membrane attached at its periphery to the walls of the cavern, installing a plurality of balloons inflated with the cushion gas in the cavern or inflating a single large bladder in the cavern with the cushion gas. With additional separation of the cavern into multiple portions, a plurality of gases can be simultaneously stored in a single cavern.