Kanmon Railway Tunnel
関門トンネル
Japan's first undersea tunnel (1942), a 3,614-meter railway passage beneath the Kanmon Strait connecting Shimonoseki (Honshu) and Moji (Kyushu). A critical wartime infrastructure achievement that maintained rail links between the two main islands. The Kanmon Strait was the primary target of the U.S. B-29 "Operation Starvation" aerial mining campaign in 1945, and the OSS planned a sabotage mission with 250 agents and 23 tons of explosives — cancelled only by Japan's surrender. A parallel pedestrian tunnel (780 m, opened 1958) allows visitors to walk beneath the strait.
Images
History
The Kanmon Railway Tunnel (関門鉄道トンネル, Kanmon Tetsudō Tonneru) is the first undersea tunnel in Japan, connecting Shimonoseki in Yamaguchi Prefecture (Honshu) with Moji in Kitakyushu, Fukuoka Prefecture (Kyushu) beneath the Kanmon Strait. The name "Kanmon" derives from the kanji of Shimonoseki (下関) and Moji (門司).
Construction began in 1936 using nearly every known tunneling technique of the era: rock tunneling by drilling and blasting, shield tunneling in soft ground, compressed-air tunneling, and the immersed-tube method with pneumatic caissons. The downbound (Kyushu-bound) tunnel was completed in November 1942, during the height of the Pacific War, while the upbound (Honshu-bound) tunnel opened in 1944. The Kyushu-bound tunnel measures 3,614 meters and the Honshu-bound tunnel 3,604 meters, with 1,140 meters running beneath the seabed. The track gauge is 1,067 mm (narrow gauge) and the electric power supply is 1,500 V DC.
The tunnel held immense strategic significance during World War II. Prior to its completion, all rail freight and passenger traffic between Honshu and Kyushu relied on ferry crossings of the Kanmon Strait — a vulnerable chokepoint. The tunnel provided a protected, all-weather rail link immune to surface naval threats, enabling the continuous flow of troops, munitions, coal, and industrial materials between Japan's industrial heartland and the military bases of Kyushu.
U.S. military intelligence identified the Kanmon Strait as "the single most vulnerable point in the enemy's shipping position." Beginning on March 27, 1945, the 313th Bombardment Wing launched Operation Starvation, an aerial mining campaign using B-29 Superfortresses. In the first phase alone (March 27 – April 12), 2,030 mines were dropped into the narrow strait, closing it for two weeks. By June 1945, maritime traffic through the strait had fallen to just 10 percent of normal volume. Over the full campaign, 12,135 mines sank or damaged 670 ships totaling over 1.25 million tons — more tonnage than all other anti-shipping efforts in the final six months of the war combined.
The Office of Strategic Services (OSS) also developed a plan to destroy both railway tunnels with 50,000 pounds (approximately 23,000 kg) of explosives, training 250 agents for the sabotage mission as part of preparations for the invasion of Japan. Japan's surrender on August 15, 1945 rendered the operation unnecessary.
After the war, the tunnel continued to serve as a vital rail artery. A parallel roadway tunnel (Kanmon Roadway Tunnel) was completed in 1958 under National Route 2, featuring a unique pedestrian passage (関門トンネル人道, Kanmon Tonneru Jindō) on its lowest level. The 780-meter pedestrian tunnel runs approximately 55–60 meters below sea level, with elevator access on both the Shimonoseki side (at Mimosusogawa Park) and the Moji side (near Mekari Shrine). Walkers can cross for free in about 15 minutes, passing a famous underwater prefectural boundary marker midway — one of the few places in the world where one can walk between two prefectures beneath the ocean floor.
The Kanmon Railway Tunnel was designated a Heritage of Industrial Modernization (近代化産業遺産) by Japan's Ministry of Economy, Trade and Industry. Today, both tunnels remain in active use, and the pedestrian tunnel is a popular tourist attraction drawing visitors who wish to literally walk beneath the sea between Honshu and Kyushu.