It makes me nervous to think that I am walking in the undersea tunnel of Kanmon Straits.

The Kanmon Tunnels are undersea tunnels that connect the Honshu side (Shimonoseki City, Yamaguchi Prefecture) and the Kyushu side (Kitakyushu City, Fukuoka Prefecture). Three Kanmon Tunnels penetrate under the seabed of the Kanmon Straits: the Kanmon Railway Tunnel for the Sanyo Main Line (conventional railway), the Kanmon Roadway Tunnel for the road (National Highway No. 2), and the Shin-Kanmon Tunnel for the Sanyo Shinkansen Bullet Train. The Kanmon Roadway Tunnel is also equipped with the tunnel for pedestrians.

The first Kanmon Tunnel connecting Honshu and Kyushu was the Kanmon Railway Tunnel, opened in 1942. It was the world’s first undersea tunnel. Construction began in 1936, and the down line was completed in 1942 during the war, while the up line began in 1940 and opened in 1944 during also the time. The total length of the tunnel is 3604 m for the up line and 3614 m for the down line, and the length of the seabed is 1140 m for both the up and down lines. The installation slope of the tunnel is 20‰ for the down line, and 22‰, a maximum of 25‰ for the up line. The up and down lines were built separately for ease of construction and as a precaution against accidents. Until the construction of the Kanmon Railway Tunnel, the Kanmon Straits had been used to transport vehicle of freight trains by Kanmon ferry, but in order to save time and effort in transshipment and strengthen transportation capacity, a proposal to increase the number of ferry boats, a proposal to construct bridge and a proposal to construct tunnel were considered. However, the large number of vessels passing through the Kanmon Straits and the strong tidal currents made it difficult to further increase the number of ferry boats. Tunnels are cheaper to build than bridges and are less likely to be bombed. Therefore, the tunnel proposal was adopted from the standpoint of national defense.

The Kanmon Railway Tunnel pass through an area of the Kanmon Straits called “O-Seto”. The Kanmon Roadway Tunnel, the Shin-Kanmon Tunnel, and the Kanmon Bridge of the expressway passes through the waters called “Hayatomo-no Seto” at the strait’s narrowest part (about 600 m wide). Because of the deep water in the Hayatomo-no-Seto and taking into consideration the performance of locomotives towing trains, the Kanmon Railway Tunnel adopted the O-Seto route so that the tunnel would not have a steep gradient. In addition, the Kanmon Railway Tunnel could not connect to the former Moji Station (now Mojiko Station) due to the topography, so it was decided to connect to Dairi Station. The former Moji Station was renamed Mojiko Station, and Dairi Station was renamed Moji Station as the gateway to Kyushu.

When the Kanmon Railway Tunnel opened (1942), DC electrification was implemented between Hatabu Station and Moji Station on the Sanyo Main Line, and EF10 class electric locomotives were introduced into this section. In 1961, when the Kagoshima Main Line between Mojiko Station and Kurume Station was electrified with AC, a dead section was created within Moji Station. Initially, it was considered to convert the inside of the Kanmon Railway Tunnel to AC electrification, but due to concerns about electricity leakage due to water leakage, etc., it was decided to remain DC, and the electric locomotives introduced in conjunction with this were the EF30 class. The EF30 class was the world’s first mass-produced AC/DC electric locomotive that adopted a stainless-steel body to prevent salt damage and was compatible with both DC and AC power sources. A total of 22 EF30 class were manufactured, one prototype and 21 mass-produced, and they were used exclusively for the Kanmon Tunnel, All EF30 class vehicles were retired by 1987 with the introduction of the successor EF81 class 300s and 400s. One EF30 01 is on display as a static preservation vehicle in front of Kanmonkaikyo Mekari Station in Mekari Park, and the front head of one EF30 03 is on display as a dynamic preservation vehicle at the Kyushu Railway History Museum.

In the Kanmon Railroad Tunnel, 600 tons of salt-containing underground water seeps out every day. The saltwater damages the rails and electrical equipment faster than above ground, so almost every day, for about three hours, train service on either the up or down lines is cancelled, and workers repair and inspect the rails and electrical systems.

At platform 5 and 6 of JR Moji Station, there are information boards with photos of the down line inside the Kanmon Railway Tunnel and an overview of the Kanmon Railway Tunnel, its history, construction, and the trains that have run through it.

The Kanmon Roadway Tunnel began in 1937 with the excavation of an exploratory shaft. However, construction was halted in 1945 due to material shortages and war damage caused by World War II. Construction resumed in 1952, and the tunnel was opened to traffic in 1958. Of the total length of 3,461 m, the undersea portion is 780 m. The tunnel is a two-lane road (National Route 2) with one lane on each side. The tunnel has a two-tiered structure, with the upper two-thirds of the cross section being for roadways and the lower one-third for pedestrians. The width of the roadway tunnel is 7.5 m, and the pedestrian tunnel is 4 m. The front and rear approach sections are structured as roadways only.

The total length of the Kanmon Pedestrian Tunnel, which is attached to the Kanmon Roadway Tunnel, is 780 meters. Due to its location directly under the roadway, the pedestrian road is also designated as National Highway No. 2. An elevator is installed at the entrance of the pedestrian tunnel, which descends to about 55 m below ground on the Shimonoseki side and about 60 m below ground on the Moji side. Pedestrians, bicycles, and motorized bicycles of 50 cc or less are allowed to use the tunnel, but since it is treated as a pedestrian-only road, traffic inside the tunnel is on the right side, and bicycles and motorized bicycles must be pushed by hand with no engine running. Tolls are free for pedestrians and 20 yen for bicycles and motorbikes bicycles of 50 cc or less. The tunnel is open from 6:00 am to 10:00 pm. There is a border marker between Yamaguchi and Fukuoka Prefectures at the midpoint of the pedestrian tunnel. It takes about 15 minutes on foot to pass through the bottom of the Kanmon Straits.

The entrance to the Kanmon Pedestrian Tunnel (Shimonoseki side) is located across National Route 9 from Mimosusogawa Park (Dannoura Ancient Battlefield Site). The adjacent Kanmon Plaza is home to the Kanmon Tunnel Museum operated by the West Japan Expressway.

The entrance to the Kanmon Pedestrian Tunnel (Moji side) is located in Mekari Park. Nearby is Mekari Shrine, famous for the Mekari Shinto ritual. Also, in Mekari Park is the Kanmonkaikyo Mekari Station of the “Retro Train” sightseeing trolley train, which connects Mekari Park and the Mojiko Retro District in 10 minutes, a distance of 2.1 km.

The Shin-Kanmon Tunnel is a railroad undersea tunnel with a total length of 18713 m and an undersea portion of 880 m, located between Shin-Shimonoseki Station and Kokura Station on the JR West Japan Sanyo Shinkansen Line. It was completed in 1975 with the opening of the Shinkansen Line to Hakata Station.

At the time of construction, the maximum speed of the Sanyo Shinkansen was 200 km/h. However, in order to accommodate high-speed operation in anticipation of 260 km/h operation, the steepest slope of the Shinkansen tunnel was set at 15‰ or less and the minimum curve radius was set at 4000 m. The completed Shin Kanmon Tunnel had a total length of 18713 m, installation slope of 18‰ as a special case, and a minimum curve radius of 3500 m. Although the undersea tunnel portion of the Shin Kanmon Tunnel is shorter than the Kanmon Railway Tunnel, the total tunnel length is five times longer in the Shin Kanmon Tunnel.

Incidentally, a gradient of 15‰ or less means that the vertical rise in height is 15 m or less when going 1000 m horizontally. The minimum curve radius is the lowest value that guarantees safety and comfort. If you extend the curve of a railway track as it is, it will become a circle, and the radius of this circle is the radius of the curve. The smaller the curve radius value, the steeper the curve.

Access to Kanmon Pedestrian Tunnel Moji side

It takes about 1 hour and 50 minutes from Haneda Airport (Tokyo) to Kitakyushu Airport. Approximately 35 minutes by Nishitetsu bus from Kitakyushu Airport to JR Kokura Station.

It takes about 2 hours and 20 minutes by Shinkansen from JR Shin-Osaka Station (Osaka) to JR Kokura Station.

Take the Kagoshima Main Line (bound for Mojiko Station) from JR Kokura Station to JR Mojiko Station for about 15 minutes and get off at JR Mojiko Station. 25 minute-walk from JR Mojiko Station.

Alternatively, at JR Mojiko Station, transfer to the Kitakyushu Bank Retro Line Kyushu Railway Kinenkan Station and get off at Norfolk Hiroba Station, or take the Nishitetsu Bus (bound for Mekari) and get off at Kanmon Tunnel Jindo-guchi bus stop, a short walk.

Access to Kanmon Pedestrian Tunnel Shimonoseki side

It takes about 1 hour and 20 minutes from Haneda Airport (Tokyo) to Yamaguchi Ube Airport. Approximately 1 hour and 15 minutes by Airport shuttle bus with reservation from Yamaguchi Ube Airport to JR Shimonoseki Station.

It takes about 1 hours and 10 minutes by Shinkansen from JR Shin-Osaka Station (Osaka) to JR Shin-Simonoseki Station.
From JR Shin-Shimonoseki Station, take the Sanyo Main Line (bound for Shimonoseki Station) for about 10 minutes and get off at JR Shimonoseki Station. 20-minute walk from JR Shimonoseki Station. Alternatively Sanden Kotsu Bus (bound for Chofu Station) from JR Shimonoseki Station to Kanmon Tonneru Jindo-guchi Bus Stop, a short walk.