The Hokenoyama Kofun does not conform to the standard keyhole shape of most large burial mounds in Japan. Its design is a distinctive scallop shape, with a large circular rear section measuring 60 meters in diameter, built in three tiers to a height of 8.5 meters, and a short rectangular frontal extension just 20 meters long and only 3.5 meters high. The whole structure stretches approximately 80 meters in length, orientated to the southeast, and is surrounded by a moat that varies from 10.5 to 17 meters wide. The exterior was covered with fukiishi -- roofing stones gathered from the nearby Makimuku River -- but no haniwa clay figures were found, placing its construction before that tradition became standard. The mound sits east of the much larger Hashihaka Kofun, within the Makimuku Kofun Cluster, collectively designated a National Historic Site in 2006.

The 1995 excavation reached the heart of the circular mound and found a stone burial chamber with walls of piled river stones and a wooden ceiling. Inside lay a massive stone sarcophagus, approximately 6.5 meters long and 2.6 meters wide, enclosing a split-bamboo-shaped wooden coffin carved from a single piece of Japanese cedar. The coffin's interior had been painted with vermillion. Decorated jar-shaped earthenware was arranged at the top of the coffin, though it had fallen inward as the wood decayed over centuries. Despite partial looting, the grave goods were extraordinary: a complete picture-and-text-belted divine beast bronze mirror, 23 fragments of a second divine beast mirror that had been deliberately broken before burial, an inner-circle floral pattern bronze mirror, and numerous bronze and iron arrowheads, swords, and tools. Pottery came from not just the local area but also the Tokai region and the Seto Inland Sea, suggesting far-reaching political or trade connections.

The mid-third-century dating of the Hokenoyama Kofun places it in one of the most debated periods in Japanese history. Chinese chronicles, particularly the Records of the Three Kingdoms, describe a powerful shamanistic queen named Himiko who ruled the kingdom of Yamataikoku during this era. Scholars have long argued over where Yamataikoku was located -- northern Kyushu and the Yamato region of Nara are the leading candidates. The Makimuku area, where the Hokenoyama Kofun sits, is central to the Yamato theory. The nearby Omiwa Shrine asserts, without physical evidence, that this tumulus is the grave of Toyosukiiri-hime, a daughter of Emperor Sujin and chief priestess of the Ise Grand Shrine. Whether royal or not, the occupant commanded impressive resources: a cedar coffin large enough for a person of high status, bronze mirrors with Chinese-influenced designs, and weapons gathered from across the Japanese archipelago. One of the bronze mirrors from the tomb is now kept at the Omiwa Shrine.

The Hokenoyama Kofun was not used once and sealed. A second wooden coffin was discovered in the rectangular frontal portion of the mound, and a simple direct burial was found in the narrow connecting section between the two halves. Most intriguingly, a second stone burial chamber exists on the west side of the circular mound -- a subsequent interment from around the end of the sixth century, three hundred years after the original burial. Someone chose to reuse this ancient mound for their own dead, grafting new significance onto a structure already centuries old. The artifacts excavated from the original burial are collectively designated a National Important Cultural Property. Today the mound sits quietly in the Hashihaka neighborhood of Sakurai, its contours softened by erosion, a road slicing through its southeast end, the vermillion coffin and divine beast mirrors long removed to museums and shrines.