Seikan Tunnel: The World’s Longest Tunnel That Dips Underwater Links Japan’s Islands

The Seikan Tunnel in Japan is the longest tunnel with an underwater section in the world, measuring a total of 53.85 kilometers (33.4 miles) in length, around 23.3 kilometers (14.5 miles) of which is under the seabed.

With the underwater section located 100 metres (328 feet) below the seabed of the Tsugaru Strait, the Seikan tunnel is also the second longest railway tunnel in the world, beaten only by the Gotthard Base Tunnel in the Swiss Alps. The longer railway tunnel in Switzerland, however, channels through mountains, not the seabed. 

The Channel Tunnel between France and the UK is the longest underwater tunnel in the world, with an undersea section that’s 38 kilometers (23 miles) long. However, the Seikan Tunnel – including sections not under the seabed – is longer overall and deeper

It took many decades to get this engineering megaproject off the ground. A decisive moment in its inception occurred in 1954, when Typhoon Marie struck Japan and sank five ferry boats crossing the Tsugaru Strait. In total, 1,430 people lost their lives. 

Shaken by the maritime tragedy, Japan started to explore safer options for mass transportation across the Tsugaru Strait. In 1955, the Japanese National Railways launched a study to see whether a tunnel beneath the channel would be feasible. 

Excavations began in 1964, but the project was riddled with mishaps. It wasn’t until 1971 that construction on the main Seikan Tunnel began. The main tunnel breakthrough was achieved in 1985 and it was finally opened in 1988. Part of the construction also involved two undersea stations, one on the coast of each island. 

All in all, the tunnel’s construction cost an eye-watering ¥689 billion (around $4.6 billion). Some estimates put the figure at ¥1.1 trillion ($7 billion), although it’s unclear where this figure came from. Either way, the project cost a hell of a lot of money. 

Fortunately, the Seikan Tunnel is well-used. It’s estimated that around 50 trains travel through the tunnel per day, including both freight and passenger services. It’s even capable of supporting Japan’s famous Shinkansen bullet trains, which have a top speed of 320 kilometers (199 miles) per hour.

The Seikan Tunnel allows passengers to travel from Tokyo to Hakodate, one of the main cities on Hokkaido, on a route called the Hokkaido Shinkanse. The journey via the Seikan Tunnel takes just over 4 hours and costs ¥23,120 ($150).

By 2030, Japan hopes to extend this route all the way to Sapporo, the capital of the mountainous northern island Hokkaido. This journey is set to take roughly 5 hours.

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