Negrón Tunnel Curiosity · Sena de Luna, La Montaña de Luna, León More than four kilometres long, the tunnel that pierces the Cantabrian watershed on the Ruta de la Plata motorway put an end to the valley's winter isolation and left the Pajares pass as a scenic alternative. Photo: Nachosan (CC BY-SA 3.0) What to see and do in Sena de Luna