Seikan Tunnel

The Seikan Tunnel is a massive dual-gauge railway tunnel in Japan. It serves as a vital link between Honshu, the main island, and Hokkaido, the northern island.

A major segment of this tunnel runs deep beneath the seabed of the Tsugaru Strait. Because of its immense scale, it holds the title of the world's longest undersea tunnel. This makes it longer than the famous Channel Tunnel, even though the Channel Tunnel has a longer undersea portion. It is also a very deep transport tunnel. Until the Ryfylke Tunnel opened in Norway in 2019, it was the deepest in the world. Today, it remains the second longest main-line railway tunnel, following Switzerland’s Gotthard Base Tunnel.

Building such a structure required a complex, multi-stage engineering process. To manage the undersea section, engineers did not just dig one hole. Instead, they excavated three separate bores with increasing diameters. They started with an initial pilot tunnel, which is a small exploratory path. Next, they dug a service tunnel to assist with operations. Finally, they excavated the main tunnel for the actual trains.

The service tunnel was periodically connected to the main tunnel using a series of connecting drifts. This layered approach helped engineers manage the immense pressure and geological challenges of the seabed.

The geology of the Tsugaru Strait presented many obstacles for the construction crews. The area is folded into a structure called a syncline, where the youngest rock is in the center. The Honshu side consists mostly of volcanic rocks like andesite and basalt. The Hokkaido side is made of sedimentary rocks, such as tuff and mudstone. The center portion contains the Kuromatsunai strata, which is a sand-like mudstone.

To avoid the danger of earthquakes, the engineers chose to route the tunnel through dense volcanic rock. However, they still faced difficulties from igneous intrusions and faults that crushed the rock. These features made the drilling and excavation procedures very complicated.

The history of the tunnel began with strategic ideas in the late 1920s. In 1946, the Ministry of Railways began feasibility studies following the end of World War II. A tragic event in 1954 helped speed up these plans. During a typhoon, five ferries sank in the strait, resulting in the deaths of 1,430 passengers. This disaster highlighted the need for a safer way to travel between islands. By 1964, the Japan Railway Construction Public Corporation was established to begin the work. Drilling officially started from both sides in 1972, beginning at Hamana on Honshu and Yunosato on Hokkaido.

Construction was an incredibly expensive and difficult undertaking. The project faced delays from the 1973 oil crisis and various drilling difficulties. It also faced challenges from tunnel flooding and a loss of market share to air travel. The total cost reached approximately ¥1.1 trillion, which was nearly 12 times the original budget due to inflation.

During the process, 34 workers lost their lives. Despite these hardships, the tunnel finally opened on 13 March 1988. To celebrate this achievement, the Japan Mint issued a special 500 yen coin featuring the tunnel.

Today, the tunnel is a critical part of Japan's transportation network. It uses dual-gauge tracks, which allow both standard-gauge and narrow-gauge trains to use the same rails. This allows the high-speed Hokkaido Shinkansen to run alongside regular freight trains. Because freight trains share the tunnel, the Shinkansen cannot always travel at its maximum design speed. However, during major holidays, freight traffic is suspended so Shinkansen trains can speed up to 260 km/h. The tunnel handles many trips every day, with up to 50 round trips if you include freight services.

Safety remains a top priority for the tunnel's operation. While two underwater stations, Tappi-Kaitei and Yoshioka-Kaitei, were closed to passengers, they still serve a vital purpose. They now act as emergency escape points for travelers. These stations are equipped with special technology to protect people during a disaster. They feature exhaust fans to remove smoke, television cameras for guidance, and thermal infrared fire alarms. They even have water spray nozzles to help manage fires. This ensures that the massive undersea passage remains a safe route for everyone traveling between the islands.