Ojo Guareña is a karst complex located in the Cantabrian Mountains of Castile and Leon, Spain, declared a natural monument by the government of Castile and Leon in 1996. It is composed of over of galleries and passages within an area of some. The limestone formation containing the system is approximately thick and sits on a massive water-resistant layer of marl. The caves were formed in the limestone by erosion sometime within the Coniacian Age. Ojo Guareña was considered the greatest karst system of the Iberian Peninsula until 2009, when a significant length of new passages was discovered in the Mortillano system. Archaeological findings in the area indicate that various caves in the Ojo Guareña system were used by humans as early as the Middle Palaeolithic up until the Middle Ages. Scientific exploration has also discovered the presence of over 180 species of invertebrates in the complex. Tourist access to the caverns is permitted, and there is a visitor's centre located in Quintanilla del Rebollar.
Features
The karst complex of Ojo Guareña consists of a massive network of over galleries and cavities, not all of which are connected, occupying an area of about 13,000 ha. The main network consists of fourteen caves that are connected with each other over six different levels of the system. The most prominent entry point is at Palomera Cave. Ojo Guareña was formed by the erosive action of the Guareña and Trema rivers, in addition to the Villamartín stream. These bodies of water run through the lower galleries for most of the year, although during flood season it is possible for the water to enter the higher galleries. Eventually, they return to the outside through karst springs at Trema, Torquilla, Torcón and Torcona, located on a lower section of Trema River. The groundwater runs through Ojo Guareña at a speed of 1.5–5 km per day, depending on whether the measurement is made during the drier summer or the flood season. The groundwater feeds an aquifer located in the hinge of the area's syncline fold.
Evidence has been found to indicate the presence of human beings in and around the caves of Ojo Guareña from at least as early as the Middle Palaeolithic up until the Middle Ages.
Palaeolithic
The oldest signs of human presence, including cave paintings and tools, in the complex of Ojo Guareña are in the cave of Prado Vargas. Excavations in 1973 located prehistoric pottery fragments. In 1986, more pottery fragments and stone tools were located, indicating a possible Neanderthal presence. In 2006, some 400 Mousterian pieces were located and the site was classified as an archaeological industry. Impressions of footprints making a round trip, accompanied by torch fragments that have been radiocarbon dated to as old as 15,600 years, indicate that the cave has been in use for millennia.
In the Cave of Kaite, paintings have been found that appear to represent deer. On the walls and the vault of the Sala de la Fuente, or Fountain Room, are paintings that depict people, animal forms, and geometric drawings. These images span a period of time between the late Neolithic and the beginning of the Bronze Age.
Protected status
On March 14, 1996, Decrees 60/1996 and 61/1996 were issued, approving the Natural Resource Management Plan for Ojo Guareña and declaring the official status of Ojo Guareña as Natural Monument of Ojo Guareña. Decree 174/1998, of September 3, 1998, regulates the composition of the Governing Board of the Ojo Guareña Natural Monument.
