The Seikan Tunnel, is a 53.85 km dual-gauge railway tunnel in Japan, with a 23.3 km portion under the seabed of the Tsugaru Strait, which separates Aomori Prefecture on the main Japanese island of Honshu from the northern island of Hokkaido. The track level is about 100 m below the seabed and 240 m below sea level. The tunnel is part of the standard-gauge Hokkaido Shinkansen and the narrow-gauge Kaikyō Line 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 Hakodate, the nearest major city on the Hokkaido side.

The Seikan Tunnel is the world's longest undersea tunnel by overall length (the Channel Tunnel, while shorter, has a longer undersea segment). It is also the second longest main-line railway tunnel after the Gotthard Base Tunnel in Switzerland, opened in 2016. Seikan Tunnel is the second-deepest transport tunnel below the sea level after the Ryfylke Tunnel. Howerver, Ryfylke has no railway.

The Seikan tunnel was opened on 13 March 1988.  Today, about 50 freight trains and 30 Shinkansen bullet trains pass through the tunnel every day, carrying more than two million tons of cargo each year. But freight trains and high-speed bullet trains are not the best of traveling companions, especially in the tunnel.

The Hayabusa Shinkansen series, a Japanese bullet train with a top speed of 199 miles per hour, has to slow down to 87 miles per hour in the tunnel to avoid disrupting the freight trains. At full speed, the wind pressure created when the two trains cross paths could cause the freight trains to spill their loads. Ways to manage this situation, and to raise the speed limit, are still being investigated.

According to Wikipedia/ atlasobscura