## 2 Caves Studied: Tourist Visit Regimen  
The El Castillo cave forms part of the karstic system of Monte Castillo (Puente Viesgo, Cantabria, Spain), where there are more than forty cavities excavated in Carboniferous limestone [18], six of them decorated during the Upper Palaeolithic, namely El Oso, El Lago, La Flecha, Las Monedas, La Pasiega, and Las Chimeneas. Monte Castillo is 385 m high and is a conical hill that dominates the left bank of the wide fluvial plain formed by the Pas river. The single entrance to the El Castillo cave is located at a point of 195 metres and is currently a large shelter as a result of the different archaeological excavations carried out since its discovery in 1903. This large shelter is protected from the exterior by a visitor reception building, while it connects to the rest of the cave through a small door for the passage of tourists in single file. The length of El Castillo cave is almost 800 m with a descent of \(-16\) m, distributed in five main halls. The more eastern hall, closer to the cave entrance, is the largest volume of the cave (30 m wide, 25 m long, and 8 m high) [19]. Huge blocks and columns occupy the central area of this hall, determining the existence of numerous compartmentalised places and secondary corridors of minor dimensions, some of them currently connected by staircases for modern tourists. The microclimatic monitoring station is situated in the north of this large hall in a small chamber, namely Hands Ceiling, with an outstanding rock panel decorated mainly with non-figurative images but also including nine outlined bison, two engraved deer, and at least 40 hand stencils [20]. The Covalanas cave is located in a steep relief of the Lower Cretaceous Urgonian limestones of Mount Pando (Ramales de la Victoria, Cantabria, Spain). The single entrance to the cave is 320 m above sea level on the right bank of the Calera River. The cave is divided in two parallel running galleries; the one on the left extending 40 m and the right hand one a straight and quasi-horizontal passage of approximately 100 m long. The width of the longest gallery ranges 3-5 m until 60 m from the entrance, and it narrows to 1-2 m in the end sector where the microclimatic monitoring station is located. The gallery has hardly any speleothems of significant size, such as obstructing the direct bidirectional airflow between the cave and the exterior. This gallery contains stunning quality paintings,  
Figure 1: Location and list of caves included in Cave of Altamira and Palaeolithic Cave Art of Northern Spain UNESCO World Heritage. * The Monte Castillo Group includes El Castillo, La Pasiega, Las Monedas, and Las Chimeneas caves. including figurative images and geometric signs, mainly located between 60 and 90 m from the cave entrance [21]. El Castillo and Covalanas are part of the seven Cantabrian caves open to the public. One of the main points of conflict is how to maintain public visits to these caves without endangering the conservation of the paintings [22]. The tourist demand for the caves has increased in the last two decades. In 2003, 64,570 visitors visited the region's rock art sites, and following their World Heritage status, this figure increased to 117,731 in 2019 [23]. Most visitors come during the high- and medium-tourist season in Spain (May to September). Ideally, a carrying capacity study should commence with the installation of instruments to establish a baseline of the cave's natural environmental conditions, free from anthropogenic alterations and prior to any tourist activity. Achieving this requires nearly a year of microenvironmental recording without human disturbance [11; 24; 25; 26; 27]. Although this particular study began after years of tourist activity and some modifications to the natural environment, the cave, with restricted access during the COVID-19 pandemic, provided a unique opportunity.