Formally named the Seto Inland Sea, the Inland Sea is the body of water separating Honshū, Shikoku, and Kyūshū, three of the main islands of Japan. It serves as an international waterway, connecting the Pacific Ocean to the Sea of Japan. It connects to Osaka Bay and provides a sea transport link to industrial centers in the Kansai region, including Osaka and Kobe. Before the construction of the Sanyō Main Railroad Line, it was the main transportation link between Kansai and Kyūshū.

Yamaguchi, Hiroshima, Okayama, Hyōgo, Kagawa, Ehime, Fukuoka, and Ōita prefectures all have coastlines on the Inland Sea; the cities of Hiroshima, Iwakuni, and Matsuyama are also located on it.

The Inland Sea region is known for its moderate climate, with a stable year-round temperature and relatively low rainfall levels: The area is often called "the land of fair weather". The sea is also famous for its periodic red tides caused by dense groupings of certain phytoplankton that result in the death of large numbers of fish.

Since the 1980s, its northern and southern shores have been connected by the three routes of the Honshū-Shikoku Bridge Project, including the Great Seto Bridge, which serves both railroad and automobile traffic.

An inland sea is a shallow sea that covers central areas of continents during high stands of sea level that result in marine transgressions. In modern days continents stand high, eustatic sea levels are low, and there are few inland seas, none larger than the Caspian Sea. Modern examples might also include the recently (less than 10,000 years ago) reflooded Persian Gulf, and the South China Sea that presently covers the Sunda Shelf.

On a geologic time scale, inland seas have been greater in extent and more common.

• A vast inland sea extended from the Gulf of Mexico deep into present-day Canada during the Cretaceous. See the Western Interior Seaway.

• At the same time, much of the low plains of modern-day northern France and northern Germany were inundated by an inland sea, where the chalk was deposited that gave the Cretaceous Period its name.

• The Amazon, originally emptying into the Pacific, as South America rifted from Africa, found its exit blocked by the rise of the Andes about 15 mya. A great inland sea developed, at times draining north through what is now Venezuela before finding its present eastward outlet into the South Atlantic. Gradually this inland sea became a vast freshwater lake and wetlands where sediment flattened its profiles and the marine inhabitants adapted to life in freshwater. Over 20 species of stingray, most closely related to those found in the Pacific Ocean, can be found today in the freshwaters of the Amazon, which is also home to a freshwater dolphin. In 2005 fossilized remains of a giant crocodilian, estimated to have been 46 ft (14m) in length, were discovered in the northern rainforest of Amazonian Peru 12-9-2005_pg6_6" target="_blank">http://www.dailytimes.com.pk/default.asp?page=story_12-9-2005_pg6_6.

• In Australia the promise of an expected inland sea was one of the prime motives of inland exploration of Australia during the 1820s and 1830s. The main champions of the theory were _Charles Sturt and John Oxley, but it had a number of other supporters. Notable sceptics included Edward John Eyre.

• The Baltic Sea is a brackish inland sea, alleged to be the largest body of brackish water in the world (other possibilities include the Black Sea). It occupies a basin formed by glacial erosion.