The Seikan Tunnel is a 53.85 km (33.46 mi) railway tunnel in Japan, with a 23.3 km (14.5 mi) long portion under the seabed. The track level is about 100 metres (330 ft) below the seabed and 240 m (790 ft) below sea level.[1] It travels beneath the Tsugaru Strait—connecting Aomori Prefecture on the main Japanese island of Honshu with the northern island of Hokkaido—as part of the Kaikyo 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 Hakodate , the nearest major city on the Hokkaido side.
The Seikan Tunnel is the deepest operational main-line rail tunnel in the world (though Guangzhou Metro Line 3's central tunnel has been longer since 2010) and the second longest after the Gotthard Base Tunnel in Switzerland. While the Seikan Tunnel is also the longest undersea tunnel in the world, the Channel Tunnel between the United Kingdom and France has a longer undersea portion.