The Shimosa Plateau sits about 30 meters above sea level, and the kofun crowd its narrow spine in dense strips. Of the 114 confirmed tumuli, the forms tell a story of evolving burial customs: keyhole-shaped mounds 20 to 30 meters long, dome-shaped mounds with diameters of 10 to 20 meters, scallop-shaped mounds, and one massive square monument. Many more may have been destroyed by road construction and urban encroachment over the centuries, and others likely remain undiscovered beneath the soil. More than 70 percent of the surviving mounds sit on prefectural land, which has helped preserve them. Haniwa -- the distinctive terracotta figures and cylinders placed on and around kofun -- have been found at only 16 of the tumuli. The vast majority of the mounds remain unexcavated, their contents still sealed beneath centuries of accumulated earth and silence.

The oldest mound in the cluster is believed to be the scallop-shaped Kofun No. 101, dated to the second quarter of the sixth century based on the style of haniwa recovered there. The later mounds concentrate in the northeast corner of the site, and among them is the Sengenyama Kofun, thought to be the last kofun ever built in all of Chiba Prefecture. This span of over a century reveals something remarkable: the people of this plateau continued building burial mounds well into the Hakuho period, even as Buddhism was establishing temples nearby. The ruins of the Buddhist temple Ryukaku-ji stand close to the cluster, evidence that the two traditions -- the old earthen burials and the new imported faith -- coexisted here, side by side, for generations before the mounds finally stopped rising.

Kofun No. 105, known as the Iwaya Kofun, dominates the cluster. It is a square-sided tumulus roughly 78 meters on each side, constructed in three ascending tiers that give it the appearance of a step pyramid -- a form startlingly reminiscent of structures from entirely different civilizations thousands of miles away. At 13 meters tall, it is the largest mound in the cluster. Inside sit two lateral-type stone-lined burial chambers built from blocks of soft limestone packed with fossilized shells. A double moat once encircled the entire structure, enclosing an area measuring 108 meters east to west by 96 meters north to south. But the entrance has stood open since the Edo period, and over those centuries, any grave goods were long ago removed. The Iwaya Kofun is a monument stripped to its architecture, impressive for its scale alone.

Some of the larger mounds in the cluster contained more than one burial chamber, suggesting reuse across generations -- families returning to the same earthen monument to inter successive members. Others were repurposed for a new burial well after their original construction. These details hint at a society where the mounds were not merely tombs but active features of the landscape, revisited and repurposed as communities changed. Artifacts recovered from excavated sites are displayed at the Chiba Prefectural Boso Mura no Fudoki no Oka Museum, a five-minute walk from the Fudoki no Oka Kitaguchi bus stop. The collection includes haniwa figures, stone chamber fragments, and other objects that give material substance to a period that left its mark primarily in shaped earth.