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. Flora and fauna A number of birds live in the area surrounding the cave complex, including the golden eagle, short-toed snake eagle, Egyptian vulture, Eurasian sparrowhawk, Eurasian eagle-owl, griffon vulture, tawny owl, and the peregrine falcon. Inside the caves there are several species of bats, such as the cave bat, greater horseshoe bat, lesser horseshoe bat, and Geoffroy's bat. Other