• - April 24, 1946: the geological study begins.[3]​.

• - September 26, 1954: the ferry train Tōya Maru sinks in the Tsugaru Strait.[3]​.

• - March 23, 1964: Japan Public Railway Construction Corporation established.[3]​.

• - September 28, 1971: Construction of the main tunnel begins.[3]​.

• - January 27, 1983: progress of the pilot tunnel.[3]​.

• - March 10, 1985: progress of the main tunnel.[3]​.

• - March 13, 1988: The tunnel opens.

• - March 26, 2016: Shinkansen services begin operating through the tunnel, regular narrow gauge passenger services through the tunnel cease.[10]​.

Prospecting began in 1946. In 1971, 25 years later, work began. In August 1982, less than 700 meters remained to be excavated. The first contact between the two sides took place in 1983.[6]​.

The Tsugaru Strait has the western and eastern necks, both 20 km long. Surveys begun in 1946 indicated that the eastern neck, more than 200 m deep, was of volcanic geology. The western neck, with a maximum of 140 m, has sedimentary rocks from the Neogene period. The western neck was finally selected because its conditions were optimal for a tunnel.[13]​.

The geology of the undersea portion of the tunnel is volcanic rock, pyroclastic rock, and Late Tertiary sedimentary rock.[14] The area is on a vertical anticline slope, meaning that the most recent rock is in the center of the stratum. It can be roughly divided into three parts: the Honshū side consisting of volcanic rocks (andesite, basalt, etc.); the Hokkaido side with sedimentary rocks (Tertiary period) and the central portion consisting of Kuromatsunai stratum (sand-like Tertiary Period).[15]​ Igneous intrusions and faults caused the rock to break and complicated the work.[13]​.

Initial geological investigations were conducted between 1946 and 1963, and involved seabed drilling, sonic testing, underwater observations (with mini-submarines), seismic and magnetic testing.[13]​.

The tunnel was drilled simultaneously from the north side and the south side. The construction was carried out on the land portion with traditional mountain tunneling techniques, with a single main tunnel.[13] But, for the 23.3 km of the undersea portion, three tunnels with increasing diameters were excavated, respectively: an initial pilot tunnel, a service tunnel and finally the main tunnel. The service tunnel is linked to the main tunnel by a series of connecting galleries at intervals of 600-1000 m.[15]​ The pilot tunnel serves the service tunnel for the 5 km of the central portion.[13]​.

Near the Tsugaru Strait, a drilling machine (TBM) was stopped after less than 2 km due to the variable density of the rock at the site, making it difficult to access for drilling.[14]​[13]​ The methods for excavation were dynamite and manual drilling.

A 2002 report by Michitsugu Ikuma described, for the undersea section, that "the tunnel structure is apparently in good condition."[16] The number of leaks has been decreasing over time, although "the risk during an earthquake increases."[16] In March 2018, at 30 years, maintenance costs amounted to 30 billion yen or US$286 million since 1999. Plans are to increase speed and provide mobile communication throughout the runway.[17]​.

Initially, only narrow gauge (1067 mm) passed through the tunnels, but the Hokkaido Shinkansen (which began construction in 2005 and whose first train crossed the tunnel on March 26, 2016) included the laying of three-lane tracks (narrow and standard 1435 mm) and connected the tunnel to the Shinkansen network, so Shinkansen trains can pass through the tunnel to Hakodate and, in the future, they will be able to do it all the way to Sapporo. The tunnel has 52 km of continuous rails without welding.[18]​ The maximum speed in the tunnel is 140 km/h on the normal track and 110 km/h on the narrow track.[19]​.

Two stations are connected to the tunnel: Tappi-Kaitei and Yoshioka-Kaitei. The stations serve as escape points for emergencies. In the event of fire or other disasters, the stations provide safety equivalent to another shorter tunnel. The effectiveness of escape exits located at emergency stations is increased by exit fans that absorb smoke, television cameras that guide passengers to the exit, thermal (infrared) alarms, and water launchers. In addition, both stations have museums detailing the history and function of the tunnel, and can be visited on special tours. Only now Tappi-Kaitei remains as a museum, Yoshioka-Kaitei was demolished on March 16, 2006 to make way for preparations for the Hokkaido Shinkansen.[20]​ The two stations were the first in the world to be built under the sea.[21][22]​.

• - Annex:Large tunnels.

• - Wikimedia Commons hosts a multimedia category on Seikan Tunnel.

• - The Seikan Tunnel, Aomori Prefectural Government, version dated May 3, 2006 on the Internet Archive.

• - April 24, 1946: the geological study begins.[3]​.

• - September 26, 1954: the ferry train Tōya Maru sinks in the Tsugaru Strait.[3]​.

• - March 23, 1964: Japan Public Railway Construction Corporation established.[3]​.

• - September 28, 1971: Construction of the main tunnel begins.[3]​.

• - January 27, 1983: progress of the pilot tunnel.[3]​.

• - March 10, 1985: progress of the main tunnel.[3]​.

• - March 13, 1988: The tunnel opens.

• - March 26, 2016: Shinkansen services begin operating through the tunnel, regular narrow gauge passenger services through the tunnel cease.[10]​.

Prospecting began in 1946. In 1971, 25 years later, work began. In August 1982, less than 700 meters remained to be excavated. The first contact between the two sides took place in 1983.[6]​.

The Tsugaru Strait has the western and eastern necks, both 20 km long. Surveys begun in 1946 indicated that the eastern neck, more than 200 m deep, was of volcanic geology. The western neck, with a maximum of 140 m, has sedimentary rocks from the Neogene period. The western neck was finally selected because its conditions were optimal for a tunnel.[13]​.

The geology of the undersea portion of the tunnel is volcanic rock, pyroclastic rock, and Late Tertiary sedimentary rock.[14] The area is on a vertical anticline slope, meaning that the most recent rock is in the center of the stratum. It can be roughly divided into three parts: the Honshū side consisting of volcanic rocks (andesite, basalt, etc.); the Hokkaido side with sedimentary rocks (Tertiary period) and the central portion consisting of Kuromatsunai stratum (sand-like Tertiary Period).[15]​ Igneous intrusions and faults caused the rock to break and complicated the work.[13]​.

Initial geological investigations were conducted between 1946 and 1963, and involved seabed drilling, sonic testing, underwater observations (with mini-submarines), seismic and magnetic testing.[13]​.

The tunnel was drilled simultaneously from the north side and the south side. The construction was carried out on the land portion with traditional mountain tunneling techniques, with a single main tunnel.[13] But, for the 23.3 km of the undersea portion, three tunnels with increasing diameters were excavated, respectively: an initial pilot tunnel, a service tunnel and finally the main tunnel. The service tunnel is linked to the main tunnel by a series of connecting galleries at intervals of 600-1000 m.[15]​ The pilot tunnel serves the service tunnel for the 5 km of the central portion.[13]​.

Near the Tsugaru Strait, a drilling machine (TBM) was stopped after less than 2 km due to the variable density of the rock at the site, making it difficult to access for drilling.[14]​[13]​ The methods for excavation were dynamite and manual drilling.

A 2002 report by Michitsugu Ikuma described, for the undersea section, that "the tunnel structure is apparently in good condition."[16] The number of leaks has been decreasing over time, although "the risk during an earthquake increases."[16] In March 2018, at 30 years, maintenance costs amounted to 30 billion yen or US$286 million since 1999. Plans are to increase speed and provide mobile communication throughout the runway.[17]​.

Initially, only narrow gauge (1067 mm) passed through the tunnels, but the Hokkaido Shinkansen (which began construction in 2005 and whose first train crossed the tunnel on March 26, 2016) included the laying of three-lane tracks (narrow and standard 1435 mm) and connected the tunnel to the Shinkansen network, so Shinkansen trains can pass through the tunnel to Hakodate and, in the future, they will be able to do it all the way to Sapporo. The tunnel has 52 km of continuous rails without welding.[18]​ The maximum speed in the tunnel is 140 km/h on the normal track and 110 km/h on the narrow track.[19]​.

Two stations are connected to the tunnel: Tappi-Kaitei and Yoshioka-Kaitei. The stations serve as escape points for emergencies. In the event of fire or other disasters, the stations provide safety equivalent to another shorter tunnel. The effectiveness of escape exits located at emergency stations is increased by exit fans that absorb smoke, television cameras that guide passengers to the exit, thermal (infrared) alarms, and water launchers. In addition, both stations have museums detailing the history and function of the tunnel, and can be visited on special tours. Only now Tappi-Kaitei remains as a museum, Yoshioka-Kaitei was demolished on March 16, 2006 to make way for preparations for the Hokkaido Shinkansen.[20]​ The two stations were the first in the world to be built under the sea.[21][22]​.

• - Annex:Large tunnels.

• - Wikimedia Commons hosts a multimedia category on Seikan Tunnel.

• - The Seikan Tunnel, Aomori Prefectural Government, version dated May 3, 2006 on the Internet Archive.