## 2 Caves Studied: Tourist Visit Regimen
This closure allowed for the collection of data that closely reflected the cave's natural state, minimising the influence of visitor activity and offering a valuable approximation of pre-tourism conditions. The calendar and opening hours of the two caves studied were initially pre-established by the Government of Cantabria ([[https://cuevas.culturalacetantabria.com/](https://cuevas.culturalacetantabria.com/)]([https://cuevas.culturalacetantabria.com/](https://cuevas.culturalacetantabria.com/)), accessed on 19 March 2025), setting as closing days for tourist visits every Monday of each week and on public holidays. The maximum quotas for each group of visitors accompanied by a guide were also established at 14 visitors/group (El Castillo) and eight visitors/group (Covalanas). In both caves, each guided visit takes approximately 45 min. Since the reopening of caves for tourism on 26 May 2020 after a forced public closure on 14 March 2020 due to the COVID-19 epidemic, these visitors per group quotas changed according to the health situation. Tables 1 and 2 summarise the monthly and daily rates of visitors to the El Castillo cave and Covalanas cave, respectively, during the monitoring period of 2020-2022. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**2020**} & \multicolumn{3}{c}{**2021**} & \multicolumn{3}{c}{**2022**} \\  & **Total** & **Mean** & **Max.** & **Total** & **Mean** & **Max.** & **Total** & **Men.** & **Max.** \\ \hline January & - & - & - & 493 & 21 & 51 & 999 & 40 & 114 \\ February & 2101 & 96 & 168 & 756 & 33 & 84 & 1679 & 67 & 128 \\ March & 886 & 81 & 124 & 887 & 35 & 114 & 2441 & 84 & 180 \\ April & 0 & 0 & 0 & 1529 & 59 & 111 & 4148 & 148 & 228 \\ May & 53 & 11 & 20 & 1382 & 53 & 113 & 3861 & 133 & 236 \\ June & 879 & 37 & 80 & 2195 & 84 & 129 & 4122 & 159 & 340 \\ July & 2168 & 80 & 93 & 3212 & 115 & 136 & 5579 & 192 & 272 \\ August & 2681 & 103 & 124 & 3183 & 118 & 145 & 5451 & 202 & 233 \\ September & 2546 & 98 & 126 & 3022 & 104 & 186 & 4156 & 154 & 221 \\ October & 1621 & 60 & 115 & 2645 & 91 & 198 & 3679 & 131 & 197 \\ November & 184 & 14 & 67 & 2101 & 78 & 191 & 2833 & 105 & 202 \\ December & 419 & 28 & 70 & 1861 & 72 & 217 & 2250 & 98 & 204 \\ \hline Annual & 13,538 & 55 & 90 & 23,266 & 72 & 140 & 41,198 & 126 & 213 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Monthly statistics of the number of visitors to the El Castillo cave during the 3-year period of 2020–2022 (data from the beginning of the environmental monitoring work in February 2020). The columns Mean and Max correspond to the average daily values and the average of the maximum daily values in each month, respectively. From 2020, marked by limitations and temporary closures to the public, until 2022, there has been a notable increase in the number of visitors to the three caves. In the El Castillo cave, there was a 72% increase in visits in 2021 (23,274 visitors) compared to 2020 (13,558 visitors from February to December), while in 2022 (41,415 visitors/year), the number of visitors tripled compared to 2020. In the Covalanas cave, visits doubled in 2021 (5196 visitors) compared to 2020 (2528 visitors from February to December), while in 2022 (7798 visitors/year), the number of visitors tripled compared to 2020. On a daily scale, there was also an increase in the average and maximum daily visits to the three caves in all months of the year compared to the same period (month) of the previous year, except for the first quarter of 2021 (low season), when restrictions on tourist activity were still in place due to the health situation. At the El Castillo cave, the annual average of average daily visits increased from 55 visitors/day in 2020 to 72 visitors/day in 2021 and 127 visitors/day in 2022. The daily visitor flow rates at the Covalanas cave changed from monthly average values of 12 visitors/day in 2020 to 19 visitors/day in 2021 and 27 visitors/day in 2022.