Seto Inland Sea
The Seto Inland Sea (瀬戸内海 or Seto Naikai), also known as Setouchi is the body of water separating Honshū, Shikoku, and Kyūshū, three of the four main islands of Japan. The region that includes the Seto Inland Sea and the coastal areas of Honshū, Shikoku, and Kyūshū is known as the Setouchi Region. It serves as a 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 San'yō Main Line, it was the main transportation link between Kansai and Kyūshū.
Yamaguchi, Hiroshima, Okayama, Hyōgo, Osaka, Wakayama, Kagawa, Ehime, Tokushima, Fukuoka, and Ōita prefectures all have coastlines on the Seto Inland Sea; the cities of Hiroshima, Iwakuni, Takamatsu, and Matsuyama are also located on it.
The Setouchi region is known for its moderate climate, with a stable year-round temperature and relatively low rainfall levels. The sea is also famous for its periodic red tides (赤潮 akashio) caused by dense groupings of certain phytoplankton that result in the death of large numbers of fish.
Since the 1980s, the sea's 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.