World’s longest undersea tunnel–The Seikan Tunnel

Posted on June 16, 2008. Filed under: Science and Technology |

The Seikan Tunnel is a 53.85 km (33.49 mile) railway tunnel in Japan, with a 23.3 km (14.5 mile) portion under the seabed. It is the longest undersea tunnel in the world. It travels beneath the Tsugaru Strait — connecting Aomori Prefecture on the Japanese island of Honshū and the island of Hokkaidō — as part of the Kaikyo Line of Hokkaido Railway Company. Although it is the longest traffic (railway or road) tunnel in the world, faster and cheaper air travel has left the Seikan Tunnel comparatively underused. Its claim to the record will be taken when the Gotthard Base Tunnel, another railroad tunnel, is completed around 2018.

In 1954, a typhoon sank five ferry boats in Japan’s Tsugaru Strait and killed 1,430 people. In response to public outrage, the Japanese government searched for a safer way to cross the dangerous strait. With such unpredictable weather conditions, engineers agreed that a bridge would be too risky to build. A tunnel seemed a perfect solution. Ten years later, work began on what would be the longest and hardest underwater dig ever attempted.

Engineers couldn’t use a tunnel boring machine to carve the Seikan Tunnel because the rock and soil beneath the Tsugaru Strait was random and unpredictable. Instead, tunnel workers painstakingly drilled and blasted 33 miles through a major earthquake zone to link the main Japanese island of Honshu with the northern island of Hokkaido. Today, the Seikan Tunnel is the longest railroad tunnel in the world at 33.4 miles in length, 14.3 miles of which lie under the Tsugaru Strait.

Three stories high and 800 feet below the sea, the main tunnel was designed to serve the Shinkansen, Japan’s high-speed bullet train. Unfortunately, the cost of extending the Shinkansen service through the new tunnel proved to be too expensive. In fact, air travel today between Honshu and Hokkaido is quicker and almost as cheap as rail travel through the tunnel. Despite its limited use, the Seikan Tunnel remains one of the greatest engineering feats of the 20th century.

The Structure

Currently, only narrow gauge track is laid through the twin tunnels, but the Hokkaidō Shinkansen project (which started construction in 2005) will include laying dual-gauge track and linking the tunnel into the Shinkansen network, so Shinkansen trains can traverse the tunnel to Hakodate (scheduled for 2015) and eventually Sapporo. The tunnel has 52 km of continuous welded rail.

Two stations are located within the tunnel: Tappi-Kaitei Station and Yoshioka-Kaitei Station. The stations serve as emergency escape points. In the event of a fire or other disaster, both stations provide the equivalent safety of a much shorter tunnel. The effectiveness of the escape shafts located at the emergency stations is enhanced by: exhaust fans to extract smoke; television cameras to route passengers to safety; thermal (infrared) fire alarm systems; and water spray nozzles.

Previously, both the stations contained museums detailing the history and function of the tunnel, and could be visited on special sightseeing tours. Now only Tappi-Kaitei remains as a museum, Yoshioka-Kaitei was demolished on March 16, 2006 to make way for Hokkaidō Shinkansen preparations.

The two stations were also the first train stations in the world built under the sea.

Some Facts

  • More than 2,800 tons of explosives were used in the construction of the tunnel.
  • One hundred sixty-eight thousand tons of steel was used in the construction of the tunnel. That’s enough steel to build four Petronas Towers!
  • The railway track runs 787 feet below the surface of the sea, making it the deepest railway line in the world.
  • During construction in 1976, tunnel workers hit a patch of soft rock with disastrous results. Water gushed into the tunnel at a whopping rate of 80 tons per minute. It took more than two months to control the flood. Luckily, no lives were lost.


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