Inland Sea, Jap. Seto-naikai, arm of the Pacific Ocean, c.3,670 sq mi (9,510 sq km), S Japan, between Honshu, Shikoku, and Kyushu islands. It is linked to the Sea of Japan by a narrow channel. The shallow sea is dotted with more than 950 islands, the largest of which is Awaji-shima. The shores of the Inland Sea are heavily populated and are part of Japan's most important industrial belt. Many industrial cities line the sea from the Osaka-Kobe complex on the east to the northern Kyushu industrial complex on the west. Many of Japan's greatest ports, including Osaka, Kobe, and Hiroshima are there. The Inland Sea is also famed for its scenic beauty and is the site of Inland Sea (Seto-naikai) National Park (255 sq mi/660 sq km; est. 1934) which includes some 600 islands and coastal segments.