This week, Dominique Similox-Tohon (Structural Geology and Tectonics Group, KULeuven) and Patrick Degryse and Philippe Muchez (both Physico-Chemical Geology, KULeuven) carried out eight drillings between the Theater and the Potters' Quarter to identify the different layers present in the subsurface and correlate them with the results of the electrical resistivity profiling (see Geological Survey, July 25-29, and Seismological Studies, July 25-29). Unfortunately, numerous drill bars of the Ramguts drill were broken during the geomorphological fieldwork and all remaining bars broke during this week. This forced us to resort to manual drilling. Nevertheless, the deepest borehole still reached a depth of 4.25m. It was carried out at the bottom of an excavated workshop area of the Potters' Quarter (1.75 m depth). Below a 1.75 m infill of dark brown sand with archaeological material and limestone fragments, several layers of green gray volcanic tuff and brown clay are identified. Drilling stopped at a very hard clay layer. The lowermost layers still contain ceramics, indicating a high deposition rate. The infill corresponds to layer 2 identified on the resistivity profiles and has an intermediate resistivity. Layer 3, absent in the borehole in the Potters' Quarter but identified in lateral boreholes, is characterized by a high resistivity and represents an old scree. The volcanic tuff layers and their weathered clays form the transition between layer 2 and 4. The brown clay of layer 4 with few ceramics has a low resistivity.