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Japanese structure became famous for connecting two large islands under the Tsugaru Strait, but its record requires an important explanation
When it comes to an underwater railway tunnel, many people first think of the Eurotunnel between France and England. But Japan holds one of the most impressive works of modern engineering: the Seikan Tunnel, a railway passage of 53.85 kilometers that connects the islands of Honshu and Hokkaido under the Tsugaru Strait.
The Japanese structure drew attention again after a report by Xataka Brasil highlighted that the largest underwater railway tunnel, when considered by the total extension of the work under a maritime environment, is not in Europe, but in Japan. However, the case has an important nuance: the Eurotunnel has the longest continuous stretch under the sea, while the Seikan is longer in total extension.
Inaugurated in March 1988, the Japanese tunnel was not created just to break records. It was conceived as an infrastructure response to a real safety problem and territorial integration, especially after serious maritime accidents in the Tsugaru Strait.
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Today, the Seikan remains strategic for passengers, cargo, and the operation of the Hokkaido Shinkansen, the bullet train that connects part of northern Japan to the rest of the high-speed rail network.
Seikan Tunnel connects Honshu and Hokkaido under the Tsugaru Strait
The Seikan Tunnel connects the Aomori region, at the northern tip of Honshu, to the island of Hokkaido, one of Japan’s most important areas for agriculture, tourism, and logistics. The crossing is under the Tsugaru Strait, a stretch of sea subject to difficult weather conditions, strong waves, and navigation risks.
According to information from the Japan Railway Construction, Transport and Technology Agency, the structure has a total length of 53.85 km and was designed with three main tunnels: a pilot tunnel, a service tunnel, and the main tunnel used by trains. This configuration helps with maintenance, safety, and drainage of a work that operates in both underground and maritime environments.
Of the more than 53 km, about 23.3 km pass under the seabed. At the deepest point, the railway is approximately 240 meters below sea level and about 100 meters below the seabed, numbers that help explain why the work is considered an engineering feat.
The construction had to face fragile rocks, fissures, high-pressure infiltrations, and flooding risk. To overcome these obstacles, methods such as advanced drilling, injection of sealing materials, shotcrete, and continuous terrain monitoring were used.
Maritime tragedy of 1954 accelerated the search for a safer connection
The idea of connecting Honshu and Hokkaido via an underground railway route gained momentum after one of the most significant maritime accidents in Japanese history. On September 26, 1954, the ferry Toya Maru sank during a strong typhoon in the Tsugaru Strait, killing over a thousand people.
According to Britannica, the disaster killed about 1,150 to 1,170 passengers and crew and became a milestone in the discussion about the reliance on ferries in that region. The tragedy was not the only reason for the tunnel, but it helped turn an old idea into a national priority.
At the time, transportation between the islands relied heavily on maritime crossings. This made passengers and cargo vulnerable to weather, delays of vessels, and the risks of storms in a complicated navigation region.
The Seikan thus emerged as a long-term solution to reduce this dependency. More than a grand work, it represented an attempt to ensure logistical continuity, safety, and railway integration between two essential parts of Japanese territory.
Comparison with the Eurotunnel shows why the record causes confusion
The fame of the Eurotunnel is understandable. It connects Folkestone in the United Kingdom to Coquelles near Calais in France and has a total length of 50.45 km. Of this total, 37.9 km are under the English Channel, making it the tunnel with the longest effectively underwater section in the world.
The Seikan, on the other hand, has a greater total length, with 53.85 km, but its section under the seabed is smaller, with 23.3 km. This is why the phrase “world’s longest underwater tunnel” can vary depending on the criteria adopted.
If the comparison considers the total length of a railway structure built to cross a maritime passage, the Japanese tunnel stands out. If the criterion is only the continuous section located under the sea, the Eurotunnel takes the first position.
This difference is important to avoid an exaggerated interpretation. The Seikan is not simply “larger than the Eurotunnel” in all senses, but it is longer in the overall construction and remains a global reference in railway tunnels under extreme maritime environments.
Work needed to be prepared for bullet trains even before receiving them
One of the most curious aspects of the Seikan Tunnel is that it was designed with the future in mind. Even initially inaugurated with conventional railway operation, the main tunnel was designed with enough space to accommodate Shinkansen trains, which are wider and faster than regular Japanese trains.
According to data from Web Japan, the Seikan was built with smooth curves and inclines precisely to allow for potential use by bullet trains. This decision showed long-term vision, as the operation of the Hokkaido Shinkansen through the tunnel only began decades later, in 2016.
The arrival of bullet trains brought another challenge: the coexistence between high-speed passenger trains and freight trains. Since the tunnel is shared, the speed of the Shinkansen in this section needs to be controlled to avoid risks related to air pressure generated inside the passage.
According to the International Union of Railways, when the Hokkaido Shinkansen line began commercial operation in 2016, the maximum speed in this shared section was limited precisely because of the presence of freight trains. This detail shows that, in a project of this magnitude, maximum speed is not just a matter of technology, but also of operational safety.
Even gigantic, the tunnel also requires constant maintenance against moisture and salt
The environment of the Seikan Tunnel is especially aggressive for equipment. The presence of high humidity, salty air, and potential leaks requires frequent inspections, drainage systems, fire detectors, and preventive renovation works.
JRTT itself reports that safety and maintenance facilities are vulnerable to moisture and salt in underwater tunnels. Therefore, the structure undergoes continuous checks and replacement of deteriorated components.
This point helps to understand why tunnels of this type do not end on inauguration day. The physical construction may be ready, but the operation depends on decades of maintenance, risk control, and technological updates.
In the Japanese case, the challenge is even greater because the structure needs to function as a national railway link. Any significant failure can affect passengers, freight, and the integration between Hokkaido and the rest of the country.
Japanese tunnel became a symbol of engineering, but also shows the limits of gigantic projects
Despite its importance, the Seikan Tunnel is also remembered for an economic discussion. When it was planned, it was expected that the railway connection would have an even greater role in passenger transport between the islands.
Over the years, aviation has become a strong competitor in long-distance routes. For many passengers, flying between Honshu and Hokkaido can be faster and, in some cases, competitive in price.
Even so, the tunnel maintains strategic relevance for railway and freight transport. Hokkaido is an important region for food, agricultural products, and tourist travel, and the railway connection reduces dependence on maritime routes subject to weather conditions.
The project also left a technical legacy. The Seikan demonstrated how engineering, safety, and long-term planning can transform a dangerous crossing into a permanent route, even when the cost and complexity are enormous.
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