Seto Inland Sea

The Seto Inland Sea, often called simply the Inland Sea, is the stretch of water that lies between Japan’s three main islands: Honshu, Shikoku, and Kyushu. It functions as a passage linking the Pacific Ocean with the Sea of Japan. Through its connection to Osaka Bay, it offers maritime access to industrial hubs in the Kansai region, such as Osaka and Kobe. Until the San'yō Main Line railway was built, this sea was the primary route for moving people and goods between Kansai and Kyushu. Eleven prefectures border its shores: Yamaguchi, Hiroshima, Okayama, Hyōgo, Osaka, Wakayama, Kagawa, Ehime, Tokushima, Fukuoka, and Ōita. Major cities along its coast include Hiroshima, Iwakuni, Takamatsu, and Matsuyama.

The area encompassing the sea and its adjacent coastal lands is known as the Setouchi region. This region enjoys a warm, dry climate with relatively little rainfall. Periodic red tides—dense blooms of certain phytoplankton—occur here, often causing large fish kills. Since the 1980s, the northern and southern shores have been linked by three bridge routes of the Honshū–Shikoku Bridge Project, including the Great Seto Bridge, which carries both trains and cars.

According to the International Hydrographic Organization’s 1953 definition, the sea’s limits are as follows: on the west, a line from Nagoya Saki in Kyushu through Uma Sima and Muture Simia to Murasaki Hana in Honshu; on the east (Kii Suidō), a line from Takura Saki in Honshu to Oishi Hana on Awaji Island, then across that island to Sio Saki and on to Oiso Saki in Shikoku; on the south (Bungo Suidō), a line joining Sada Misaki in Shikoku and Seki Saki in Kyushu.

Geographically, the sea stretches about 450 kilometers from east to west, while its north-south width ranges from 15 to 55 kilometers. The water is generally shallow, averaging 38 meters deep, with a maximum depth of 105 meters. Hydrologically, it is not a true inland sea—it is not an epeiric body like Hudson Bay or an isolated endorheic basin like the Caspian Sea. Instead, it is a marginal sea, a division of the Pacific Ocean partly enclosed by islands, archipelagos, and peninsulas (in this case, the Japanese Home Islands), and open to the ocean at the surface. The Naruto Strait links the eastern part of the sea to the Kii Channel, which leads to the Pacific. The western part connects to the Sea of Japan via the Kanmon Straits and to the Pacific through the Bungo Channel.

Different sections of the Seto Inland Sea have their own Japanese names. For instance, Iyo-nada (or Iyonada) is the strait between Ehime, Yamaguchi, and Ōita prefectures in the west; Aki-nada (Akinada) is the open water west of the Geiyo Islands near Hiroshima; and Suō-nada (Suonada) lies between Yamaguchi prefecture and Suō-Ōshima. Numerous straits run between the major islands, and smaller ones connect the sea to other seas or the Pacific. Nearly 3,000 islands dot the sea. The largest is Awaji-shima, followed by Shōdo-shima; many smaller islands are uninhabited.

Major islands include, in the eastern part: Awaji Island, Shōdo Island, the Ieshima Islands, Naoshima Islands, Shiwaku Islands, and Yumeshima. In the central part: the Geiyo Islands (including Ōmishima and Innoshima), Itsukushima (commonly called Miyajima), the Hinase Islands, and the Kasaoka Islands. In the western part: Suō-Ōshima, the Uwakai Islands, and the Hashira-jima Islands.

More than 500 marine species inhabit the sea. Examples include the ayu (an amphidromous fish), horseshoe crabs, finless porpoises, and great white sharks, which have occasionally attacked people here. Historically, whales entered the sea to feed or breed, but due to whaling and pollution, they are now rarely seen.

During the last ice age, sea levels were lower. As the ice melted, seawater flooded a basin between the Chūgoku and Shikoku mountains, forming the present-day Seto Inland Sea. From ancient times, it served as a key transport route between its coastal areas—including what is now the Kansai region and Kyushu—and also between Japan and other countries, such as Korea and China. Even after major highways like the Nankaidō and San'yōdō were built, the sea remained a vital transport corridor. Records show that foreign emissaries from China and Korea sailed these waters. The importance of maritime traffic led to the rise of private navies, known in documents as suigun (water armies) or simply pirates. Often seen as public enemies, they were frequently granted self-governance due to their power. During the feudal period, suigun controlled most coastal areas. Notable suigun lords included the Kono clan in Iyo Province (now Ehime Prefecture) and the Kobayakawa (later Mōri) clan in Aki Province (now part of Hiroshima). In the 12th century, Taira no Kiyomori attempted to move Japan’s capital from Kyoto to the coastal village of Fukuhara (modern Kobe) to boost trade with China’s Song dynasty, but the move failed, and Kyoto soon regained its status. Later, the Battle of Yashima took place off the coast of present-day Takamatsu.