If you were to build a huge castle, where would you source the stone for your fortifications?

You’d need the big stuff – taller than a man. You could find some in the mountains, but how would you transport them tens of kilometers overland? That alone is already a tall order. So, what if said mountain was right next to the sea?

You would take the stone down the mountain to boats on the shore. After that, the ocean becomes your highway.

The Bisan Archipelago was crucial in the provision of stone for Osaka Castle, due in part to its islands essentially being “floating mountains.” Granite is widely distributed in and around the Inland Sea. Shodoshima, Hon Island, Kitagi Island – their floating mountains were quarries unto themselves.

Mt. Takanbo on Hon Island shows similar traces of development – arrow marks on the mountain’s surface. Engravings in the rock. When the Tokugawa shogunate built Osaka Castle during the Edo Period, the Hosokawa Clan quarried rock from this place, then sent it off to Osaka.

However, freighting by sea was far from easy. At first glance, the Seto Inland Sea appears calm; but complex currents flow between the islands, their directions changing at all times. To cross such waters, they needed veterans who knew the sea.

Sailors involved with maritime traffic since ancient times lived in the Shiwaku Islands. They weaved between the islands as they read the tides and wind. They were artists at the rudder, and they called the Shiwaku Islands home.

In short, the Bisan Archipelago had three features: It was rich in stone. It was close to the sea. And it had skilled sailors to navigate the waters.

Thanks to these conditions, large stones could be quarried and ferried far across the sea to Osaka. Rock, naturally, cannot move itself. Nevertheless, human beings ingeniously linked mountains and oceans to create a highway.

Said highway doesn’t end at the islands, but continues beyond the sea.