Read more about the line in English Wikipedia:
Hokkaido Shinkansen
The line uses the Seikan tunnel under the Tsugaru-Kaikyō strait. The tunnel is 53.9 km, of which the submarine part is 23.3 km.
The deepest point is 240 meters below sea level, 100 meters below sea bottom.
The tunnel was originally used by the Kaikyō Line, opened 1988. Since March 26, 2016, the tunnel is used by both the Hokkaidō Shinkansen and
the freight trains that still use the Kaikyō Line. Therefore, the tunnel and the adjacent sections are dual-gauge, both 1067 mm and 1435 mm.