The Seikan Tunnel is a 53.85 km (33.46 mi) dual gauge railway tunnel in Japan, with a 23.3 km (14.5 mi) long portion under the seabed. The track level is about 100 m (330 ft) below the seabed and 240 m (790 ft) below sea level.
It extends beneath the Tsugaru Strait — connecting Aomori Prefecture on the main Japanese island of Honshu with the northern island of Hokkaido — as part of the standard gauge Hokkaido Shinkansen and the narrow gauge Kaikyō Line portion of the Hokkaido Railway Company (JR Hokkaido)'s Tsugaru-Kaikyō Line.
The name Seikan comes from combining the on'yomi readings of the first characters of Aomori, the nearest major city on the Honshu side of the strait, and Hakodatethe nearest major city on the Hokkaido side.
The Seikan Tunnel is the world's longest tunnel with an undersea segment (the Channel Tunnel, while shorter, has a longer undersea segment). It is also the second deepest and the second longest main-line railway tunnel after the Gotthard Base Tunnel in Switzerland opened in 2016.
According to en.wikipedia