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.