Seikan Tunnel (south portal) - longest undersea tunnel in the world

The Seikan Tunnel is a 53.85-kilometre (33.46 mi) railway tunnel in Japan, with a 23.3-kilometre (14.5 mi) long portion under the seabed. Track level is about 140-metre (460 ft) below seabed and 240-metre (790 ft) below sea level. It travels beneath the Tsugaru Strait—connecting Aomori Prefecture on the Japanese island of Honshū and the island of Hokkaidō—as part of the Kaikyo Line of Hokkaido Railway Company. The name Seikan comes from combining the on'yomi readings of the first characters of Aomori and Hakodate, the nearest major city on the Hokkaido side.

It is the longest undersea tunnel in the world, although the Channel Tunnel between the United Kingdom and France has a longer undersea portion, and also the deepest rail tunnel in the world. It is the longest rail tunnel in the world, although the longer Gotthard Base Tunnel in Switzerland is under construction and scheduled to open to traffic in 2017.