The Akyu site sits at 904 meters elevation on a ridge between the Aku and Ohaya Rivers, covering 56,000 square meters. Excavations revealed a settlement that evolved through three distinct cultural phases, providing an unusually clear window into how Jomon communities transformed over millennia. In the earliest phase, the Sekiyama period, approximately 30 round pit dwellings and eight square pillar dwellings, likely elevated granaries, were arranged around an oval central plaza measuring 30 by 70 meters. This dates to the early Jomon period, roughly 4,000 to 2,500 BCE. In the middle Kurohama phase, the configuration shifted to a horseshoe arrangement, with structures built directly on the ground surface rather than in pits. The final Moriso phase, from approximately 1,500 to 300 BCE, produced the site's most spectacular feature: the massive stone circle constructed on the outer circumference of that same central plaza, with residential areas pushed beyond its perimeter.

The stone circle at Akyu is staggering in scale. Its donut shape measures 120 meters at its widest and 90 meters at its narrowest, filled with an estimated 200,000 fist-sized river rocks hauled from nearby waterways. At the center stands a pillar 1.34 meters tall, assembled from 24 large and small andesite stones. Paired with two flat stones positioned to the northeast, this central monument was surrounded by eight smaller stone circles, each about a meter in radius and composed of roughly ten stones. The entire complex was ringed by an estimated 700 burial sites. The sheer labor involved in transporting and placing hundreds of thousands of stones suggests a community with both the organizational capacity and the spiritual motivation to undertake monumental construction, a picture that challenges older assumptions about Jomon people as simple hunter-gatherers.

The Akyu ruins do not exist in isolation. The area surrounding Mount Yatsugatake has one of the densest concentrations of Jomon settlement sites anywhere in Japan. Immediately to the north lies the Togariishi ruins, and to the south the Idojiri ruins, both major settlements from the same broad period. Until the discovery of the Sannai-Maruyama site in distant Aomori Prefecture, Akyu was the largest known Jomon site in the country. This clustering along the Yatsugatake volcanic range was no accident. The mountains provided obsidian for tool-making, rivers supplied fish and building stone, and the elevation offered a temperate climate with abundant forest resources. The Jomon people who lived here were part of a network of communities linked by trade, ritual, and the shared geography of this volcanic corridor. The site was designated a National Historic Site of Japan in 1979.

The decision to bury the Akyu ruins rather than relocate or destroy them was a remarkable act of preservation through concealment. The expressway now passes overhead, its drivers unaware of the ancient settlement beneath their wheels. Some of the artifacts recovered during the 1975 excavation, including distinctive Jomon pottery and stone tools, are displayed at the Nagano Prefectural History Museum in the city of Chino and at a local museum in the village of Hara. The site itself, however, remains sealed underground, waiting. Whether future generations will choose to unearth and display the ruins, or leave them protected in their engineered tomb, remains an open question. For now, the largest stone circle in Japan sleeps beneath the hum of traffic on one of the country's busiest highways, a 4,000-year-old monument to communal effort hidden just meters below the modern world.