Geodetic Survey for Long Undersea Tunnel Construction

Japan consists of four large islands and other small islands. Hokkaido is the second largest island. The planning of an undersea tunnel connecting the main island of Honshu to Hokkaido was started by Japan National Railways in 1939 and a geological investigation was started in 1946. The western route of the strait was determined from the results of these investigations in 1964 and in the same year the Japanese Railway Construction Agency started to drive the undersea pilot tunnel from both sides of the strait. Excavation of the main tunnel was started in 1971 and completed in 1985, and the tunnel has been used by the railway since March 1988. The tunnel extends for about 54 km and the undersea part is about 23 km long.