TWENTY-THREE KILOMETERS UNDER THE SEA

This article presents details of the construction of the Seikan Tunnel linking two of the four main Japanese islands, Honshu and Hokkaido. The Tsugaru Strait is only navigable by ferry during good weather, and the linking of the islands by an undersea tunnel provides not only safe passage, but also greater access to the remote and relatively unpopulated areas of Hokkaido. Another reason the government supported this project was for national security, since Hokkaido lies close to the Soviet Union and the Tsugaru Strait provides the Soviet Pacific fleet its only access to the Pacific Ocean. The extreme length, volcanic activity, and environmental considerations made the design and construction of the Seikan Tunnel one of the more fascinating events in tunnel engineering history. The design, completed in 1971, called for a tunnel 53.83 km (33.4 mi) long with an undersea portion of 23.3 km (14.5 mi). The design accommodated both Shinkansen (Bullet Train), which required wide tracks, and conventional trains with narrow tracks. Excavation of the main tunnel began in May 1972. Because of complicated geological conditions, progress was much slower than originally estimated. The tunnel was opened to rail service in 1988. The tunnel is about 100 meters (322 ft) beneath the sea bottom at its midpoint.