The Aizkorri-Aratz Natural Park is the second-largest natural park in the Basque Country, extending over 16,000 hectares, in the Aizkorri-Aratz Mountain Range. Included in the park are the highest summits (Aitxuri, 1,551 m) in the Cantabrian-Mediterranean watershed, which together form a massif that is a centre for Basque mountaineering. Declared a natural park in 2006, the area contains limestone ridge-and-valley and siliceous intrusion biomes.