Akyū ruins, Archaeological site in Hara, Japan.

The Akyū ruins are an archaeological site on a 904-meter ridge between the Aku and Ohaya rivers in Hara, Nagano, Japan. Excavations revealed 30 round pit dwellings and eight square pillar dwellings from the early Jōmon period, 20 pit dwellings from the middle Jōmon period, and a stone circle with a major diameter of 120 meters made of about 200,000 river rocks. The site was backfilled and the Chūō Expressway was built over it.

Discovered in 1975 during construction of the Chūō Expressway, the site was excavated and then backfilled so the road could be built above the buried remains. It was designated a National Historic Site in 1979. Three phases span the Jōmon period: early Jōmon pit dwellings and square pillar structures, middle Jōmon pit dwellings and graves, and a late-phase stone circle.

The people who lived here buried their dead in organized groups around a central open space, suggesting a community with shared rituals. This arrangement is still readable on the ground today and gives visitors a direct sense of how the settlement was organized socially.

Most of the objects found during excavations are kept in nearby museums rather than on site, so it is worth planning a museum visit if you want to see the actual finds. The grounds themselves show the outlines and physical remains of ancient structures directly in the earth.

A large stone circle made from around 200,000 river rocks, each roughly the size of a fist, was built on the site. It ranks among the largest known stone arrangements of its kind in Japan, and its exact purpose has never been fully explained.