Puerto de la Sia is a mountain pass at an elevation of 1.229m (4,032ft) above the sea level, located on the boundary of Cantabria and Castile and Leon regions, in northern Spain.
Located in the Cantabrian Mountains, the road to the summit, also known as Portillo de la Sia, is paved but narrow in parts. It’s called CA-665 on the Cantabria side and BU-571 on the Castile and León side. Open to traffic throughout the year, may be closed for short periods in winter when the weather is bad.
The road is pretty steep, hitting a 10.0% through some of the ramps. The pass is 21.0 km long, running south-north from Espinosa de los Monteros, a small village, towards CA-265 road. The summit hosts an abandoned mountain refuge.
A minor narrow unpaved road east of the summit links the pass with the savage Picón Blanco pass at an elevation of 1.512m (4,960ft) above the sea level. The road is 4.8 km long. 4x4 vehicles only. It’s usually impassable in winters.