Stone is still quarried today on Nishijima, one of the Ieshima Islands. For the people living around these waters, the sea provided more than fish. The islands themselves were also a valuable source of stone, which was quarried and shipped to destinations across Japan.

But why are there so many islands scattered across this sea? To find the answer, we have to look millions of years into the past, to the geological forces that shaped this region. Around three million years ago, enormous masses of rock beneath the earth pressed against one another, creating folds in the landscape, almost like wrinkles in a piece of cloth. The areas that sank formed broad stretches of water known as “nada,” such as Harima-nada, while raised areas created narrow channels known as “seto,” or straits. The name “Seto Inland Sea” itself reflects this complex geography.

At that time, however, this was not yet a sea. Around 20,000 years ago, during the last Ice Age, the area was a vast plain crossed by rivers. As the ice gradually melted and sea levels rose, water from the Pacific slowly entered from both sides. It was around 10,000 years ago that the sea finally reached Harima-nada, the waters you are crossing today. This was one of the last parts of the Seto Inland Sea to take its present form.

Look at the islands outside the window. What you see are the tops of mountains and hills that remained above the rising water. Ten thousand years may seem unimaginably long to us. But in the history of the earth, it was only a moment ago.

The region’s complex geography also creates powerful tidal currents. The Naruto, Kurushima, and Kanmon Straits are known for three of Japan’s strongest tidal currents, and all three are found within the Seto Inland Sea. These currents circulate nutrients and oxygen through the water, creating an environment rich in marine life. The Seto Inland Sea’s reputation as a treasure trove of seafood owes much to the geography that shaped it. This natural abundance may also help explain why Bozejima has flourished as an island of fishermen.

People also recognized the beauty of these islands and calm waters when the area became part of Japan’s first national park. The landscape itself was considered a treasure worth protecting. Take another moment now to look out the window.