Underground storage caverns are widely used for the bulk storage of petroleum products, in particular, crude oil. The caverns are accessed through a casing in a borehole down to the cavern. The lower end of the casing opens into an upper region of the cavern termed the chimney. The chimney provides a transition from the casing into the cavern body. The invention presents a process of injecting a gas into the well while measuring the gas pressure and optionally measuring the volume of injected gas. The gas drives down an interface between the gas and hydrocarbon liquid. By monitoring the rate of change of the gas pressure, and detecting a sudden decrease in the rate of change, it can be determined when the interface has been driven down to the region immediately below the bottom of the casing at the upper end of the chimney.