Strait, northwestern Pacific Ocean.
It extends from the Sea of Japan (East Sea) to the open ocean between the islands of Honshu and Hokkaido , Japan. It is 15–25 mi (24–40 km) wide. The Seikan Tunnel linking Aomori and Hakodate cities runs beneath the strait. Completed in 1988, it is the longest railway tunnel in the world; 14.5 mi (23.3 km) of its 33.5 mi (53.9 km) length lie under the strait.