Simplon Pass, a pass in the Alps of southwestern Switzerland. It reaches a maximum elevation of almost 6,578 feet (2,005 m) and forms part of a principal route linking the Rhone Valley in Switzerland and the Po Valley in Italy. The pass was seldom used until the Middle Ages, when it became a trade route for merchants from Milan. The first carriage road over the Simplon was built early in the 1800's. Snow usually closes the pass from December until May.
The 12.3-mile (19.8-km) Simplon Tunnel, which runs beneath Monte Leone from Brig (Switzerland) to Iselle (Italy), is one of the world's longest railway tunnels. It consists of two adjacent tubes, the first completed in 1905, the second in 1922.