Instability issues in underground cultural heritage sites

Many sites of cultural heritage present part, or all, of their development underground. In some cases, it is precisely the underground sector to host the features of greater cultural and historical interest, thus being the most remarkable section of the site for visitors and scholars. However, entering underground sites, especially after paying a fee for access, requires these are safe and do not present any element potentially harming workers and visitors. To ascertain such requirements, prevention studies and researches are needed, aimed at identifying the possible instability features within the cave system and at their surroundings as well, in order to preliminary evaluate the possibility of failures in the rock mass. This article summarizes the main activities to carry out at this goal, which should represent the first step for any attempt of opening underground sites of cultural and historical heritage to the public. After discussing the main characters of cultural heritage sites with underground development, and their importance in different parts of the world and for many past civilizations, the article examines the main features observed at Taya Cave, in Japan, in one of the case studies considered within the framework of the bilateral project JSPS/CNR “Damage assessment and conservation of underground space as valuable resources for human activities use in Italy and Japan” (2018/2019). Taya Cave, a religious site excavated by Buddhist monks as a meditation place, consists of several corridors and meditation chambers, distributed over three different levels, and partly affected by instability signs.