One of the reasons Sakata Port, located at the mouth of a river, prospered as an origin point for westbound trading ships is thanks to the existence of Tobishima Island, which acted as its outer port. Tateiwa, the large rock off the island’s southeast point, is made of a hard volcanic rock called rhyolite; this terrain helped form a natural harbor. In some years, as many as over 500 kitamaebune trading ships stopped at port in Tobishima Island.