Ojo Guareña Cave (Spain)

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 90 kilometres (56 mi) of galleries and passages within an area of some 13,850 hectares (34,200 acres). The limestone formation containing the system is approximately 100 metres (330 ft) 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. It is also the 24th longest cave 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.