## 1 Introduction  
The conservation and protection of tourist caves require appropriate management tools based on knowledge of the temporal evolution and spatial variation in the environmental parameters of the subterranean environment. Environmental impact may occur because tourist caves require a physical modification of natural passages, installation of lighting, pathways, platforms, and related infrastructure. Their related effects on the physical environment of the cave constitute a challenge for management that aims to properly evaluate the relative vulnerability of each cave passage [1].  
Monitoring air temperature and CO\({}_{2}\) concentration with adequate resolution and temporal extension is essential to perform a basic characterisation of the state and environmental dynamics of a cavity, as well as to analyse the destabilisation of the subterranean environment in the face of anthropogenic changes of a certain magnitude [2]. Both parameters play a key role in the air exchange process with the external atmosphere that involves the introduction of nutrients and the dispersion of airborne bacteria and fungi, which could potentially colonize and biodegradable the rock panels with prehistoric paintings and engravings [3; 4; 5]. In this regard, the knowledge on the confinement state of caves, evidenced by long-term continuous monitoring, helps managers to establish the best strategies to improve their conservation properties [6].  
The main tool for karst management is the definition and delimitation of protection areas and, in the case of tourist caves, the establishment of an environmentally sustainable visitor regime. There are numerous examples of tourist caves under continuous environmental monitoring, but in many cases, the analysis of the microclimatic destabilisation generated by continuous visits over time is usually addressed once the cave has been opened to tourism or after a long history of visits, which prevents us from knowing its environmental characteristics in a natural regime. In these cases, the detailed monitoring of basic microclimatic parameters and under different visitor flow rates and occasional closures to the public is essential to obtain baseline data to define numerical tools for the sustainable management of the cave in question [7]. These tools focus on defining the carrying capacity of each tourist cave, understood as the maximum flow of visitors in a cave that will produce changes in a microclimatic control parameter within a natural fluctuation range [8; 9; 10; 11; 12; 13; 14]. Since caves are considered stable habitats, medium-term microbiological monitoring has also been used to understand temporal and spatial carrying capacity for sensitive underground sites [3; 15; 16; 17].  
This study presents a characterisation of the environmental dynamics of two of the main tourist caves with parietal art in Cantabria--El Castillo and Covalanas--both included in the UNESCO Word Heritage site Cave of Altamira and Palaeolithic Cave Art of Northern Spain (Figure 1) but with a very different pattern of visitation on an annual and daily basis. Based on the records obtained during three annual cycles, the evolution of the temperature and gaseous composition of the underground air in each cave was studied in relation to seasonal changes in the local meteorology, as well as the interannual evolution of anthropogenic impacts on the underground microclimate under different regimes of daily visits. The objective of this study is to determine the carrying capacity of the two caves. This aims to prevent changes in key environmental parameters (air temperature and CO\({}_{2}\) concentration) that exceed their natural daily and/or seasonal variations, thus avoiding cumulative effects that could lead to the irreversible progression of corrosion and biodeterioration.