Defining the Optimal Ranges of Tourist Visits in UNESCO World Heritage Caves with Rock Art: The Case of El Castillo and Covalanas (Cantabria, Spain)

The Cantabrian region, located in north Spain, is home to many caves with parietal art, some of them included on the UNESCO World Heritage list, such as El Castillo and Covalanas. These two caves are currently open to tourism and boast an exceptional archaeological heritage that includes magnificent examples of Palaeolithic cave art. Through a multiyear research project (2020–2022) sponsored by the Government of Cantabria, a precise characterisation of the environmental dynamics of each cave under different meteorological contexts was carried out, as well as an evaluation of the evolution of the impacts of anthropic origin on the underground microclimate under different degrees of influx of visitors on an interannual scale. We aimed to unravel the effects of daily visitor flow on cave environmental stability and offer well-defined recommendations to harmonise conservation priorities with public accessibility based on sustainable tourism management. Once the microclimatic control parameters for the conservation of the paintings, engravings, and supporting rock, such as temperature and CO2 concentration in the air, were assessed under different seasonal meteorological conditions, a standardised graphic method was implemented based on the frequency distribution of the variations in each parameter, grouped according to the different increasing ranges of daily visits. With this method, it is possible to evaluate, probabilistically and in percentage terms, the microclimatic destabilisation of the cave generated by each group of visitors, taking as a reference the daily variation ranges of each control parameter under natural conditions, i.e., during the days or periods of time in which each cave remained closed to tourist visits. The recommended values of maximum visitor capacity for each cave, in terms of average monthly daily visitor numbers, have been set at 60 and 15 visitors/day for El Castillo and Covalanas caves, respectively. Based on these results, the cave managers are carrying out, from May 2024, a progressive adaptation in the tourist management of these caves until they are fully adapted to the environmental sustainability parameters defined in this study.