Origins of the Seto Inland Sea
The Seto Inland Sea is the body of water between the Japanese main islands of Honshu, Shikoku, and Kyushu. Often referred to simply as the Inland Sea, it is a relatively calm waterway that has been crucial in trade, travel, and warfare since ancient times. But to find out how the sea was formed, one must look much further back in time.
Some 3 million years ago, activity along the Median Tectonic Line, Japan’s longest fault line, formed mountain ranges north and south of where the Inland Sea is now. The sea flowed over the flat terrain in between, which remained exposed to tectonic activity. Subsequent land uplift and volcanic eruptions over more than a million years repeatedly changed the shape of the sea and even dried it out, until it eventually assumed its current topography approximately 1.2 million years ago.
Though its geological foundations are old, the Inland Sea as we know it today took shape relatively recently. It was completely devoid of water during the most recent ice age, but when Earth’s atmosphere warmed again approximately 7,000 years ago, the sea level on Japan’s Pacific coast rose by up to 100 meters. The Inland Sea filled with water again, and its mountainous terrain was transformed into an archipelago of some 3,000 islands.