The world's longest railway tunnel is set to open in just over a week - after some 17 years of construction. Measuring 35.4-miles in length, the Gotthard Base Tunnel (GBT) is 7,545 feet below the Gotthard massif, cutting through the Swiss Alps. The tunnel was first conceived in sketch-form in 1947 but construction began 17 years ago. It consists of two single-track tunnels connecting Erstfeld (Uri) with Bodio (Ticino) and passing below Sedrun (Graubünden).