The Pajares Base Tunnel is a twin pair of under construction long railway tunnels below Puerto de Pajares pass in the Cantabrian Mountains, Spain. The tunnels will be on the AVE line from Valladolid to Gijón but could be used also by freight trains in the future. The tunnels are affected by major water leakage, which currently delays their completion. The tunnels are currently expected to be completed in 2020, and open to passenger traffic by 2021. Although originally both tunnels were to be fit for standard gauge high-speed rail only, the plans changed mid-construction to be dual gauge with Iberian gauge for cargo use.