Stretching beneath the icy waters of northern Japan, this tunnel stands as one of the most ambitious engineering projects ever completed. At 33.4 miles long, the Seikan Tunnel is the world’s longest undersea railway tunnel and forms a permanent rail link between Japan’s main island of Honshu and the northern island of Hokkaido. 14.3 miles of the tunnel run directly beneath the seabed of the Tsugaru Strait, with trains travelling about 328ft below sea level.

Construction took decades of planning and hardship, and when the tunnel finally opened in March 1988, it carried a total price tag of roughly £5.2 billion, which is reportedly almost 12 times the original budget. The idea of a fixed link between the two islands first emerged in the early 20th century, but momentum grew after a tragic ferry disaster in 1954, when a typhoon sank five vessels and killed over 1,400 people. 

The incident exposed the dangers of relying solely on sea transport and strengthened calls for a safer, weather-proof connection.

Work officially began in 1971, with engineers boring in from both sides of the strait. The geology beneath the seabed proved extremely challenging, combining volcanic rock, sedimentary layers and active fault zones.

Flooding, cave-ins and technical setbacks were frequent, and 34 workers lost their lives during construction.

To manage the risks, engineers first created a narrow pilot tunnel, followed by a service tunnel, before finally excavating the main railway bore.

Conventional drilling and blasting methods were used for much of the work after tunnel-boring machines proved unsuitable for the unstable rock conditions.

Two underground stations, Tappi-Kaitei Station (Honshu side) and Yoshioka-Kaitei Station (Hokkaido side) were also built beneath the sea to serve as emergency escape and safety points.

When it opened, the Seikan Tunnel instantly became a global landmark. It surpassed the Channel Tunnel in total length and was, for several years, the deepest transport tunnel in the world.

Today, it carries both high-speed Shinkansen trains and freight services, forming a vital part of Japan’s rail network.