Location of the Tsugaru Strait in Japan
Connecting the islands
Hokkaido by a fixed link had been considered since the
Taishō period (1912–1925), but serious surveying commenced only in 1946, induced by the loss of overseas territory at the end of World War II and the need to accommodate returnees. In 1954, five ferries, including the
Tōya Maru, sank in the Tsugaru Strait during a typhoon, killing 1,430 passengers. The following year,
Japanese National Railways (JNR) expedited the tunnel investigation.
 Also of concern was the increasing traffic between the two islands. A booming economy saw traffic levels on the JNR-operated Seikan Ferry double to 4,040,000 passengers/year from 1955 to 1965, and cargo levels rose 1.7 times to 6,240,000 tonnes/year. In 1971, traffic forecasts predicted increasing growth that would outstrip the ability of the ferry pier facility, which was constrained by geographical conditions.
In September 1971, the decision was made to commence work on the tunnel. A
Shinkansen-capable cross section was selected, with plans to extend the Shinkansen network.
 Arduous construction in difficult geological conditions proceeded. Thirty-four workers were killed during construction.
 On 27 January 1983, Japanese Prime Minister
Yasuhiro Nakasone pressed a switch that set off a blast that completed the
pilot tunnel. Similarly on 10 March 1985, Minister of Transport
Tokuo Yamashita symbolically bored through the main tunnel.
The success of the project was questioned at the time, as the 1971 traffic predictions were overestimates. Instead of the traffic rate increasing as predicted to a peak in 1985, it peaked earlier in 1978 and then proceeded to decrease. The decrease was attributed to the slowdown in Japan's economy since the first oil crisis in 1973 and to advances made in air transport facilities and longer-range sea transport.
The tunnel was opened on 13 March 1988, having cost a total of
¥1.1 trillion (US$7 billion) to construct, almost 12 times the original budget, much of which was due to inflation over the years.
 Once the tunnel was completed, all railway transport between Honshu and Hokkaido used it. However, for passenger transport, 90% of people use air due to the speed and cost. For example, to travel between
Sapporo by train takes more than nine hours, with several transfers. By air, the journey is three hours and thirty minutes, including airport access times. Deregulation and competition in Japanese domestic air travel has brought down prices on the Tokyo-Sapporo route, making rail more expensive in comparison.
Hokutosei overnight train service began after the completion of the Seikan Tunnel, and a later and more luxurious
Cassiopeia overnight train service was often fully booked. Both were withdrawn following the commencement of
Hokkaido Shinkansen services in March 2016, with freight trains being the only regular service utilising the narrow gauge line since that time.
Shinkansen trains operate through the tunnel to
Shin-Hakodate-Hokuto Station in
Hakodate, connecting Tokyo and Shin-Hakodate-Hokuto stations in four hours and two minutes, at a maximum speed of 140 km/h (85 mph) within the tunnel and 260 km/h (160 mph) outside it, and 320 km/h (200 mph) to the south of Morioka.
 It is expected that by 2018 one daily service will be run at 260 km/h (160 mph) through the tunnel. The final stage is proposed to open to
Sapporo Station in 2031 and is proposed to shorten the Tokyo-Sapporo rail journey to five hours. The Hokkaido Shinkansen will be operated by
- 24 April 1946: Geological surveying begins.
- 26 September 1954: The train ferry
Tōya Maru sinks in the Tsugaru Strait.
- 23 March 1964: Japan Railway Construction Public Corporation is established.
- 28 September 1971: Construction on the main tunnel begins.
- 27 January 1983: Pilot tunnel breakthrough.
- 10 March 1985: Main tunnel breakthrough.
- 13 March 1988: The tunnel opens.
- 26 March 2016: Shinkansen services commence operation through the tunnel, regular narrow gauge passenger services through the tunnel cease.