Source of the Nalón Nature · Caso, Nalón, Asturias Image of Caso, not of the place itself. The longest river in Asturias springs from the Fuente La Nalona, in the heights of the parish of Tarna, inside the Redes park; from that thread of water it runs more than one hundred and thirty kilometres to the Cantabrian Sea. Photo: McBodes (CC BY 3.0) What to see and do in Caso