瀨戶內海
During the last ice age the sea level was lower than today. After the ice age, sea water poured into a basin between the Chūgoku mountains and Shikoku mountains and formed the Seto Inland Sea as we know it today. From ancient times, the Seto 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'yōdō, the Seto Inland Sea remained a major transport route. There are records that some foreign emissaries from China and Korea sailed on the Seto Inland Sea.