The Inland Sea of Seto is 450km (280 miles) long from east to west. The width from south to north varies from 15 to 55 km (9.3 to 34 miles). In most places, the water is relatively shallow. The average depth is 37.3 metres (122 feet); the greatest depth is 105 metres (344 feet).
The Naruto Strait connects the eastern part of the Inland Sea to the Kii Channel, which in turn connects to the Pacific Ocean. The western part of the Inland Sea connects to the Sea of Japan through the Kanmon Straits and to the Pacific through the Bungo Channel.
Each part of the Inland Sea has a separate name in Japanese. There are also many straits located between the major islands, as well as a number of smaller ones that pass between islands or connect the Inland Sea to other seas or the Pacific.
Almost 3,000 islands are located in the Inland Sea, including the larger islands Awajishima and Shōdoshima. Many of the smaller islands are uninhabited.
Major islands in The Seto Sea are:
Eastern part: Awaji Island, Shōdoshima, Ieshima Islands, Naoshima Islands, Shiwaku Islands.
Central part: Omishima, Innoshima, Itsukushima (popularly known as Miyajima), Hinase Islands, Kasaoka Islands.
Western part: Suooshima, Uwakai Islands, Hashirajima Islands.
Over 500 marine species are known to live in the Inland Sea. Examples are the ayu, an amphidromous fish, the horseshoe crab, the finless porpoise, and the great white shark, which has occasionally attacked people in the Inland Sea.
It is believed that in the last ice age the sea level was lower than today. After the ice age, sea water poured into a lower part between the Chūgoku mountains and Shikoku mountains and formed the Inland Sea as we know it today.
From ancient times, the Inland Sea served as a main transport line between its coastal areas, including what is today the Kansai region and Kyūshū. It was also a main transport line between Japan and other countries, including Korea and China. Even after the creation of major highways such as the Nankaidō and San'yodo, the Inland Sea remained a major transport route. There are records that some foreign emissaries from China and Korea sailed on the Inland Sea.
The coastal area of the Inland Sea is one of the most industrialized sections of modern Japan. Besides Osaka, Kobe, and Hiroshima, some other major industrial cities are Kurashiki, Kure, Fukuyama, and Ube in Honshū, and Sakaide and Niihama in Shikoku. Innoshima is also known for its ship factory.
The main industries are steel production, ship construction, and since the 1960s, oil refining and oil-derived production.
Thanks to the moderate climate and beautiful landscape, fishing, agriculture, and tourism bring a lot of income to the area as well.
The coastal area of the Inland Sea is one of the most famous tourist destinations in Japan. Even before Japan opened to foreigners in the middle of the 19th century, the sea's beauty was praised and introduced to the Western world by those who visited Japan.
Its coastal area, except for Osaka prefecture and a part of Wakayama prefecture, was appointed the Inland Sea National Park on March 16, 1934, as one of three oldest national parks in Japan.
