Japan’s Seikan Tunnel is 33 miles (53 km) long and connects the main Japanese island of Honshtii with the northern island of Hokkaido.
The tunnel extends for 14.5 miles (23 km) under water.
The Seikan Tunnel is comparatively underused because air travel is faster and cheaper.
The tunnel was opened on 13 March 1988, and cost a total of ¥538.4 billion (US$3.6 billion) to build.
The Seikan Tunnel is also the deepest rail tunnel in the world, and is part of the Kaikyo Line of the Hokkaido Railway Company.