Abstract: This invention relates to a system for monitoring, in a cavity in the earth, the position of, or changes in the position of, an interface between two immiscible liquids of different densities. In accordance with the invention, two pipes, one positioned within the other to form a space between them, extends into the cavity in the earth. A probe is mounted on the outer wall of the inner pipe and is vertically positioned near the interface to be monitored. The probe includes an emitting source and a detector-receiver and readings are taken of signals received by the detector after emission by the source and at least partially reflected by at least one of the liquids. From the readings the position of or change in interface may be computed since the signal reading will vary as a function of the relative position of the probe and the interface. The invention may be applied to the monitoring of excavations of underground salt cavities.