The Seikan Tunnel spans 53.85 km between the islands of Honshu and Hokkaido, with 23.3 km running beneath the Tsugaru Strait. Main construction began in 1971, and the tunnel officially opened on March 13, 1988, after a project that cost the lives of 34 workers. The tunnel's tracks pass about 100 meters below the seabed and 240 meters below sea level. At the time, no country had yet built a submarine tunnel of this scale. During peak construction, 3,000 workers were inside the tunnel at once, enduring temperatures of 35 degrees Celsius and humidity above 90% near the excavation site. They worked in three shifts, day and night. The challenges of the underground environment were immense, with teams having to deal with volcanic rock, unstable tectonic plates, and the risk of earthquakes. Four major water intrusions halted the work, each requiring months to seal. Landslides, floods, and various accidents caused the 34 fatalities. The idea of a fixed link between Honshu and Hokkaido was not new. Geological studies had begun as early as 1946, but a major disaster in 1954 shifted the project from an engineer’s dream to a national priority. On September 26, 1954, a typhoon caused the ferry Toya Maru to capsize near Hakodate, killing 1,153 people, with other ships on the route also sinking that night, leading to a total of 1,430 deaths. This tragedy highlighted the need for a safer connection between the islands. In May 1964, a reconnaissance tunnel was excavated in Yoshioka, Hokkaido, and main construction began in 1971. The tunnel’s name combines parts of two cities: "Sei" from Aomori and "Kan" from Hakodate. The main tunnel was finally bored on March 10, 1985, three years before the tunnel opened. At the time of its opening, air travel was more popular than rail for travelers between Tokyo and Sapporo, with a flight taking about 1 hour 30 minutes, compared to over 9 hours by conventional train. Encyclopedia Britannica notes that air transport became much more practical, and the tunnel did not carry as many passengers as expected. However, rail has made a comeback. Since March 26, 2016, the Shinkansen high-speed rail line has run through the tunnel, and conventional trains no longer carry passengers. Now, the journey from Tokyo to Shin-Hakodate-Hokuto takes 4 hours and 2 minutes, with a maximum speed of 140 km/h inside the tunnel and 260 km/h outside. The high-speed line stops at Hakodate, leaving Sapporo out of the network. Freight, however, continues to use the tunnel: the Kaikyo line became a freight-only line after 2016. Double-gauge tracks allow Shinkansen and freight trains to share the same tunnel. About 30 trains pass through the tunnel daily on the passenger side. For many years, the Seikan Tunnel held the title of the world’s longest railway tunnel until the Gotthard Base Tunnel in Switzerland reached 57.1 km in 2016. It remains longer than the Channel Tunnel, which measures 50.45 km. The Seikan is also the longest tunnel with a submerged section, according to Fun Japan, a tourist website. A part of the tunnel is open to visitors: the underwater station Tappi-Kaitei, closed to passengers in 2013, now serves as an emergency evacuation point and a tunnel museum. Visitors can descend 140 meters below sea level to see where workers once excavated. The ferry tragedy of 1954 has not been forgotten, but since the Shinkansen arrived, the Aomori-Hakodate link has seen a resurgence in usage as an economic option, offering a crossing of three to four hours.