Multi-Sensor Monitoring of Underground Heritage Sites: A Case Study of the Dragon’s Den Cave and St. Kinga’s Chapel in Poland

Long-term structural health monitoring of underground heritage sites is challenging because of high humidity, limited accessibility, complex geological conditions, and aggressive saline environments. This study presents the implementation and long-term assessment of an integrated multi-sensor telemetric monitoring system deployed in two historically significant sites: Dragon’s Den Cave beneath Wawel Castle and St. Kinga’s Chapel in the Wieliczka Salt Mine. The system combines displacement sensors, strain measurements on rock anchors, and microclimate monitoring to assess structural behavior and support the early detection of potentially hazardous changes. Monitoring was conducted for four years in the cave and two years in the salt mine. Structural measurements were analyzed together with environmental data to distinguish temperature- and humidity-related sensor responses from actual deformation. Periodic geodetic surveys provided independent verification of the telemetric measurements. Both structures remained stable throughout the observation periods. Telemetric measurements recorded changes of up to 0.16 mm at the monitored extensometer locations, while geodetic surveys indicated cumulative displacements of up to approximately 2.6 mm horizontally and 2.4 mm vertically. The results demonstrate that integrating structural and environmental sensing with periodic geodetic verification enables reliable long-term monitoring under demanding underground conditions. The proposed minimally invasive and scalable approach supports preventive conservation and safety management of underground heritage structures.