Researchers trying to unravel the mystery of who was buried in the late fifth-century Inariyama burial mound in Gyoda, Saitama Prefecture, have a new puzzle to solve after finding a hitherto hidden chamber in the structure.
The keyhole-shaped tumulus was originally encircled by a moat, denoting it was the final resting place of an individual of very high rank.
In 1968, researchers uncovered a sword blade with a gold-inlaid inscription at the site. Called Kinsakumei tekken, the blade is designated as a national treasure and believed to be among the oldest examples of Japanese script in the nation. The inscription refers to King Wakatakeru, who is assumed to have been the Emperor Yuryaku in the fifth century.
The Asahi Shimbun
Initially, it was assumed that the owner of the sword and the individual buried in the mound were one and the same. But the discovery of another chamber deeper underground and thought to be the original burial site suggests the mound was built for someone else.
The find, made with radar technology, was a joint project undertaken by Tohoku University and the Museum of the Sakitama Ancient Burial Mounds, an arm of the Saitama prefectural government responsible for the Inariyama mound and eight other such "kofun" clustered in the city.
The 73.5-centimeter-long blade, found in the rear circular section of the structure, was in a chamber of small rocks about 1 meter deep. The chamber, located slightly off-center, was fashioned from fist-sized rocks and next to one made of clay.
Researchers also found a bronze mirror, a very precious artifact in the fifth century, military items and metal fragments from horse harnesses. No human remains were found.
The inlaid characters on the sword only came to light in 1978 after the artifact underwent X-ray imaging. The inscription states that the sword was made in 471 by an individual named Wowake who served King Wakatakeru as the head of his royal guard.
An aerial view of the Inariyama burial mound in Gyoda, Saitama Prefecture (Shiro Nishihata)
The reference to King Wakatakeru had a profound impact as it confirmed the existence of an individual of the same name who appears in classical Japanese histories, such as "Nihon Shoki" (The Chronicles of Japan) and "Kojiki" (Records of Ancient Matters).
Given that the entire site is designated as having national historical significance, it is not possible for researchers to simply dig where they want. For this reason, Motoyuki Sato, a professor of applied electromagnetics at Tohoku University's Center for Northeast Asian Studies, used radar in November to investigate the rear circular section.
The tumulus is 120 meters long. The rear section measures 62 meters at its widest point.
That study picked up a shadow, indicating a lens-shaped structure 4 meters long and 3 meters wide, with a maximum thickness of 1 meter. The chamber lies about 2.5 meters below the surface. There also appears to be a part extending beyond the shadow.
Researchers from Tohoku University use radar technology to investigate the interior of the Inariyama burial mound in Gyoda, Saitama Prefecture. (Takuya Kawasaki)
Based on the location and depth, the chamber is thought to be the original burial site. The small rock and clay chambers found nearby may have been created for later generations.
Among intriguing questions that the discovery raises is whether the owner of the sword is the same individual who was first laid to rest there. There is no agreement even on whether the entombed individual came from eastern or central Japan. The discovery could also shed light on how the ancient Yamato kingdom functioned.