The Gotthard Base Tunnel (GBT; , , , ) is a railway tunnel through the Alps in Switzerland. It opened on 1 June 2016 and full service began on 11 December 2016. With a route length of , it is the world's longest railway and deepest traffic tunnel and the first flat, low-level route through the Alps. It lies at the heart of the Gotthard axis and constitutes the third tunnel connecting the cantons of Uri and Ticino, after the Gotthard Tunnel and the Gotthard Road Tunnel.