Geology

The Cueva del Asno (Donkey Cave)

The Cueva del Asno (Donkey Cave) is situated on the left bank of the Duero River, in the vicinity of Los Rábanos (Soria). Access to the cave is via the southern exit of the village. Permission must be obtained in advance to cross the Sinova dam. You will find a track heading east which will lead you to the cave. This route takes you up the river through a scrubland area known as the Bosque de Sinova, which is dominated by European red pine (Pinus sylvestris) and maritime pine (Pinus pinaster) together with specimens of holm oak (Quercus ilex).

The Cueva del Asno is a natural underground formation that has developed on limestone from the Upper Cretaceous period, between 100 and 66 million years before the present era. The action of water on the rock has given rise to important formations of stalactites, stalagmites and limestone columns. The cave comprises a large entrance hall and two large galleries running parallel to each other, forming a labyrinthine network of tunnels exceeding 1 km in length.

The route from the main entrance begins at the Paseo de las Hormigas and continues through several rooms and passages, including the somewhat narrow Sima del Perro and the Sala del Confesionario, which houses large karst formations. The Cueva del Asno displays several characteristics that indicate the cave has been inactive for a significant period. These include the presence of numerous speleothems, such as columns and concretions, as well as the absence of gours, stepped dikes arising from pools and siphons, and flooded depressed sections. These observations suggest that the cave is in a process of dissolution and collapse. In the dark corners of the museum, specifically in the room of El Museo, many bats have established a roost. Over the years, they have fertilised the floor with guano, as well as other animal species such as spiders and cave insects.

Archaeological evidence indicates that the Cueva del Asno has served as a refuge for various settlers of the Alto Duero since the Bronze Age. Its location provided protection against climatic and external adversities. The main entrance and the first exit feature different materials, which simulate a large hall illuminated by light filtering in from a side opening. The discovery of pottery and iron fragments, grave goods and the skeletal remains of prehistoric animals, including bulls (Bos taurus) and deer (Cervus elaphus), has provided valuable insights into our ancestors' lives.

It is important to note that entering a cave of this nature requires preparation and the necessary equipment. The area is not illuminated, there are slippery sections and there is a risk of asphyxiation in areas such as the Alto Duero Gallery due to the low oxygen concentration. Despite the cave's relatively easy accessibility, we must stress that caving remains a dangerous activity, even for those who practise it on a regular basis.