The Seikan Tunnel is the world's longest undersea tunnel, and the second deepest and the second longest main-line railway tunnel after the Gotthard Base Tunnel in Switzerland opened on 2016.
The Seikan Tunnel is the world's longest undersea tunnel, although the Channel tunnel has a longer undersea section. It is also the second deepest and the second longest main-line railway tunnel after the Gotthard Base Tunnel in Switzerland opened on 2016.