Castile and León is a region of Spain and is large and sparsely populated. It covers the northern half of the Inner Plateau, and is surrounded by mountain barriers. The Cantabrian Mountains lie to the North, the Sistema Central to the South and the Sistema Ibérico to the East. Most of the region is drained by a single river, the Duero (Douro River) flows west toward the Atlantic Ocean.
The most spectacular karst complex of the region is Ojo Guareña. So far 110 km of galleries have been surveyed. The complex of Coniacian limestones on top of impermeable marls is the second-largest karst area of the peninsula.
The region has underground sites along the border in the mountain ridges, there are no sights in the central plain. Most sites are in the Cantabrian Mountains. Along the western border there was mining and so there are two show mines.