A conducting cavity in an underground coal burn was remotely probed from the surface with a wideband electromagnetic induction system. The cavity, located near Hanna, Wyoming, was produced by underground gasification of a 30 foot thick subbituminous coal seam ranging in depth from 300 to 400 feet. The cavity filled with saline ground water after the burn. The wideband loop-loop system employed pseudo-noise and cross-correlation techniques to produce a transient time response in the field. Additional computer processing produced normalized 3-dimensional signature maps in both the time and frequency domains.