Haru Inland Sea

The Haru Inland Sea (春内海), also known as Haruuchi or often shortened to Inland Sea, is the body of water in Yamatai separating Harukuni and Akitsukuni, two of the five main islands of the Naichi archipelago. The Inland Sea serves as a waterway, connecting the Hokkai Sea with the Sea of Yashima. It connects to the Izumi and Musashi Bays and provides a sea transport link to industrial centres in the Harukuni heartlands, and is one of the busiest waterways in the world.

The Haruuchi region is known for its moderate climate, with a stable year-round temperature and relatively low rainfall levels. The sea is also famous for its periodic red tides caused by dense groupings of certain phytoplankton that result in the death of large numbers of fish. Located in the Haru Inland Sea are the Oki Islands, which hold major cultural importance to the Yamataian people.

Two major undersea tunnels and one bridge project enable land vehicles to cross the Haru Inland Sea. Numerous ferry services also operate in the area several times a day.