The nature of the massif is essentially calcareous and results in a surface landscape with amazing landforms, in which the karst shapes are common, with karrens, sinkholes, crevices... Moreover, it forms a subterranean landscape with more than 200 caves and chasms that have been catalogued and that have a remarkable geological, paleontological, speleological and biological value. Thus, the Pozalagua cave is an obligatory stop. It was discovered in 1957 and its importance rests in an exceptional phenomenon resulting in eccentric stalactites, that instead of growing downwards they grow in any direction. The cave is situated 2 km from the Ranero neighbourhood in the Karrantza Harana/Valle de Carranza and fitted out for tourist tours. Likewise, we must mention the Torca del Carlista, one of the biggest cavities in Europe, which is more than 400 metres long, 250 metres wide and 90 metres high.
There are several routes signed for walks going through the communities of Karrantza Harana/Valle de Carranza and Trucios and through the natural area. It is also interesting to visit the natural resources of Encinar de Sopeña, Hayedo de Balgerri, Turberas de Salduero, Zalama, megalithic monuments of Karrantza Harana/Valle de Carranza (itinerary from the necropolis to La Cabaña and Cotobasero), close monumental villages such as Balmaseda, the Avellaneda Assembly House and Karpin Abentura.