The Seikan Tunnel is the longest tunnel in the world that passes under the sea. The archipelago of Japan

The Seikan Tunnel in Japan holds the record for being the longest tunnel with an underwater portion globally. It spans a total distance of 53.85 kilometers (33.4 miles), with about 23.3 kilometers (14.5 miles) passing under the seabed.

The Seikan tunnel, situated under the seabed of the Tsugaru Strait at a depth of 100 meters (328 feet), is the second longest railway tunnel globally. Only the Gotthard Base Tunnel in the Swiss Alps is longer than it. Contrary to the seabed, the longer railway tunnel in Switzerland passes over mountains.

The Channel Tunnel, connecting France and the UK, has the distinction of being the world’s longest subaqueous tunnel, spanning a distance of 38 kilometers (23 miles) into the sea. Nevertheless, the Seikan Tunnel, which includes parts both above and below the seabed, has a greater total length and depth.

It took many decades to start this colossal engineering endeavor. An important turning point in its beginning took place in 1954, when Typhoon Marie hit Japan and caused the sinking of five ferry vessels that were crossing the Tsugaru Strait. A total of 1,430 individuals perished.

In the wake of the maritime disaster, Japan initiated an investigation into more secure alternatives for mass transit over the Tsugaru Strait. In 1955, the Japanese National Railways initiated an investigation to determine the viability of constructing a tunnel under the waterway.

The excavation began in 1964; however, the project was plagued with several setbacks. The building of the main Seikan Tunnel did not commence until 1971. The primary tunnel excavation was successfully completed in 1985, and it was subsequently inaugurated in 1988. Additionally, the building project included the establishment of two subaquatic stations, one situated along the coastline of each island.

Overall, the building of the tunnel incurred a staggering cost of ¥689 billion (about $4.6 billion). The estimated amount is ¥1.1 trillion ($7 billion); however, the source of this number is uncertain. In any case, the initiative incurred substantial expenditure.

Thankfully, the Seikan Tunnel is heavily used. Approximately 50 trains, including both freight and passenger services, traverse the tunnel on a daily basis. The system is capable of accommodating Japan’s renowned Shinkansen bullet trains, which can reach a maximum speed of 320 kilometers (199 miles) per hour.

The Seikan Tunnel facilitates transportation for people traveling from Tokyo to Hakodate, a prominent city located in Hokkaido, along a route known as the Hokkaido Shinkansen. The travel duration via the Seikan Tunnel is somewhat more than 4 hours, with a fare of ¥23,120 ($150).

Japan aims to expand this link to Sapporo, the capital of Hokkaido, a hilly northern island, by 2030. The duration of this trip is around 5 hours.