Definition of Seikan tunnel

The Seikan Tunnel (青函トンネル Seikan Tonneru or 青函隧道 Seikan Zuidō) is a 53.85 km (33.49 mile) railway tunnel in Japan, with a 23.3 km (14.5 mile) portion under the seabed. It travels beneath the Tsugaru Strait — connecting Aomori Prefecture on the Japanese island of Honshū and the island of Hokkaidō — as part of the Japan Railways Kaikyo Line. Although it is the longest railway tunnel in the world, faster and cheaper air travel has left the Seikan Tunnel comparatively underused.