Kanmon Straits in the context of "Yamaguchi Prefecture"

Fukuoka Prefecture (福岡県, Fukuoka-ken; Japanese pronunciation: [ɸɯ̥.kɯꜜ.o.ka, -kɯ.o.kaꜜ.keɴ]) is a prefecture of Japan located on the island of Kyūshū. Fukuoka Prefecture has a population of 5,109,323 (1 June 2019) and has a geographic area of 4,986 km (1,925 sq mi). Fukuoka Prefecture borders Saga Prefecture to the southwest, Kumamoto Prefecture to the south, and Ōita Prefecture to the southeast.

Fukuoka is the capital and largest city of Fukuoka Prefecture, and the largest city on Kyūshū, with other major cities including Kitakyushu, Kurume, and Ōmuta. Fukuoka Prefecture is located at the northernmost point of Kyūshū on the Kanmon Straits, connecting the Tsushima Strait and the Seto Inland Sea across from Yamaguchi Prefecture on the island of Honshu, and extends south towards the Ariake Sea.

Kitakyushu (北九州市, Kitakyūshū-shi; Japanese pronunciation: [kʲi̥.ta.kʲɯꜜː.ɕɯː, kʲi̥.ta.kʲɯː.ɕɯꜜː.ɕi]) is a city located in Fukuoka Prefecture, Japan. As of June 1, 2019, Kitakyushu has an estimated population of 940,978, making it the second-largest city in both Fukuoka Prefecture and the island of Kyushu, after the city of Fukuoka. It is one of Japan's 20 designated cities, one of three on Kyushu, and is divided into seven wards.

Sited at the northern tip of Kyushu, Kitakyushu was formed in 1963 from a merger of municipalities centered on the historic city of Kokura, and its name literally means "North Kyushu". It is located on the Kanmon Straits, separating the island from Honshu, across from the city of Shimonoseki in Yamaguchi Prefecture. Kitakyushu and Shimonoseki are connected by numerous transport links including the Kanmon Bridge and the Kanmon Tunnels (Roadway, Railway, and Shin-Kanmon).

33°57′57.4″N 130°57′47.2″E / 33.965944°N 130.963111°E / 33.965944; 130.963111The Shin-Kanmon Tunnel (新関門トンネル, shin-kanmon tonneru) is a 18.713-kilometre (11.628 mi) long undersea railway tunnel under the Kanmon Straits connecting Shin-Shimonoseki Station and Kokura Station.

At the time of opening in March of 1975, it was the longest railway tunnel in Japan before being overtaken by in 1988 the opening of the Seikan Tunnel. It is also the longest tunnel section on the entire Sanyo/Tokaido Shinkansen line.