The so-called <i>Stagnone Pontillo</i> is an ancient underground construction dug out in a rocky mass near Licata in Agrigento district (Sicily). It dates back to the pre-Hellenistic age and its first destination had presumably a religious origin. The graffiti scratched through the plaster, which still partially covers the inner walls, were identified as ancient inscriptions belonging to different linguistic groups: Phoenician, Libyan, and Iberian. The causes of alteration of the materials (stone and plaster) are investigated by thermographic methods, humidity determination, x-ray diffraction, and chemical analysis, and suggestions are given about procedures that should be adopted for the preservation and restoration of the monument. The work has been performed in co-operation with the Superintendence of Archaeological Monuments of Agrigento.