Matienzo is the name of a village and a karst depression some 25 km southeast of Santander in Ruesga municipality, Cantabria, northern Spain. The limestone around Matienzo is riddled with caves (up to 58.6 km in length) which have been extensively explored over the last 50 years by Spanish and British cavers. The total length of cave passage explored, as of December 2016, is 377 km.