Seikan Tunnel, Japanese Seikan Tonneru, undersea tunnel linking Japan’s main island of Honshu with the northern neighbouring island of Hokkaido. The Seikan Tunnel is the longest tunnel in the world; it is 33.4 miles (53.8 km) in length, 14.3 miles (23.3 km) of which lie under the Tsugaru Strait that separates Honshu from Hokkaido. The tunnel contains a rail line, and the building of the tunnel was sponsored by the Japanese National Railways. Construction of the tunnel began in 1964 and was completed in 1988; the digging employed as many as 3,000 workers at one time and took 34 lives in all because of cave-ins, flooding, and other mishaps.