A geoelectrical survey has been performed at the Mousterian (Palaeolithic) site of Grotta Reali (Molise Region, central Italy), which is one of the most important archaeological locations in the region, where Neanderthal fossils have been brought to light. An array of dipole–dipole profiles was designed in order to explore the nature of some archaeological levels found in a mostly fine-grained waste matrix along the wall of a travertine open quarry, very close to the Grotta Reali, which is a natural cavity of karst origin. Data analysis utilized a three-dimensional tomography approach based on the concept of occurrence probability of anomaly sources in the electrical resistivity distribution. The three-dimensional probability tomography allowed the geometry of an occluded portion of the prehistoric cavity to be delineated, and a vertical, elongated high resistivity block to be fully imaged inside this part of the cavity. Subsequent excavations have confirmed the inner geometry of the cavity, and identified a travertine column correlating with the high-resistivity block delineated by the three-dimensional geoelectrical tomography. Copyright © 2009 John Wiley & Sons, Ltd.