The “Piano dell’Acqua” sinkholes (San Basile, Northern Calabria, Italy)

The occurrence of sinkholes not directly related to karst has been determined in several areas worldwide in recent years. These phenomena may be particularly dangerous to humans and infrastructure due to their subtle origin and need to be carefully examined. In this work, we describe five sinkholes recently identified in a hilly setting of Southern Italy characterized by Pliocene conglomerate and sand, and variously dislocated by tectonic structures. The sinkholes were examined by different methods (interpretation of multi-temporal aerial photos, geological, geomorphological and geophysical surveys). An historical analysis was performed to collect and critically evaluate information regarding the age of the phenomena. Based on this information, it can be conjectured that two of the five sinkholes developed during the 2000–2001 winter; two of the remaining probably originated during the 70s; the last one opened sometime between February 2001 and November 2007. Based on such chronology, attempts have been performed to identify the likely triggers, through hydrological and seismic analyses. In both cases, no immediate correlation could be found. The origin of the studied phenomena remains uncertain, and may be related to sub-cutaneous erosion, in an area that is renowned to be rich in groundwater. Local changes in the water table, related to climate and/or man-induced activities, may have triggered the development of the cover suffusion-type sinkholes. The present work highlights the potential for this type of phenomena to occur in geological settings without the direct presence of soluble rocks at the surface, a condition quite common in Southern Italy, and the need to carry out further studies in order to better comprehend their mechanisms of origin and successive evolution, and to properly evaluate the related hazard.