時空を消して歴史を見る：古墳の謎Erasing Time and Space to See History

Osaka’s southern-central city of Sakai, where I was born and raised, lies at the midpoint along the Yamato River, with Osaka City on the north side and Izumi-Otsu and Kishiwada, the former Izumi Province, on the south side. The area where I first lived was near Otori Taisha, a shrine with a long history, though this shrine is now incorporated into Sakai City, it was originally part of Izumi Province. Later, my parents moved to a new residential district in Sakai, leaving behind the older historical area, but because my grandmother’s house was located in the old town of Sakai, I often visited there. Among the many memories, what stands out most vividly to me now are the burial mounds known as kofun.

The kofun in Sakai are now registered as UNESCO World Heritage sites, but looking back, a 2021 report by the Agency for Cultural Affairs stated that there are as many as 159,953 kofun sites across Japan. Taking into account those destroyed unknowingly during the nationwide large-scale development of the high economic growth period, as well as those still undiscovered, the actual number is believed to be even higher.

Examining the numbers by prefecture reveals some fascinating patterns. The prefecture with the most kofun is Hyogo, adjacent to Osaka, with 18,707, followed by Tottori, which flourished during the Yayoi period on the Sea of Japan side, with 13,505, and then Kyoto with 13,103. At first glance, Osaka Prefecture, where I live, appears low with only 3,428. Yet the distinguishing feature of Osaka’s kofun is their immense size.

Meanwhile, neighboring Nara Prefecture has 9,663 kofun. As Nara was once called Yamato, it is historically rich in heritage, and there must have been a historical progression of kofun construction moving toward the court that once existed in Nara. This remains under investigation, but with countless conflicting theories, it is difficult to determine precisely which routes influenced the kofun of Yamato.

Crucial to tracing the development of kofun is Okayama Prefecture on the Seto Inland Sea side, which has 11,880 kofun. The region around present-day Okayama City and Kurashiki was originally one of the largest areas where people who had crossed the sea gathered. Considering the main routes of arrival from the Korean Peninsula, the 10,759 kofun in Fukuoka also become significant, suggesting a route leading through the Seto Inland Sea via Hiroshima and Okayama, then onward to Hyogo, Osaka, and Nara. However, the sheer number of kofun does not necessarily indicate development; rather, it reflects differences in how each region valued burial mounds. The larger the kofun, the larger the bonds among the people who built it must have been.

When visiting my grandmother’s house, I often went to Hotate Shrine, an important local shrine. Behind it lies the tomb of Emperor Hanzei, and further beyond stands the immense Daisenryo Kofun, better known as the Mausoleum of Emperor Nintoku. These are examples of zenpo-koen-fun, or keyhole-shaped mounds, a unique type of kofun. Yet nationwide, there is great diversity—round kofun, square kofun, and others—reflecting significant regional variations in burial culture. Some kofun in Osaka are built openly, while others are tucked away in mountainous areas.

What has long fascinated me is the orientation of the kofun. For the large kofun that were designated during the Meiji era as imperial mausoleums of past emperors, detailed investigation remains restricted by the Imperial Household Agency, leaving many aspects unclear, including the direction in which the deceased were buried. The Agency’s official stance has been that investigating the tombs of emperors, who are considered divine, would be disrespectful, but this is only a surface explanation. The underlying issue is that excavation would inevitably reveal numerous artifacts showing exchange with the continent, raising difficult questions about the framework of the story told until now.

In terms of burial orientation, west or east can be interpreted within traditional views of life and death, but many kofun in Sakai face north-northeast, which remains unexplained. One possible reference comes from Ainu burial customs. According to the excellent research book Ainu no Haka (“Ainu Graves”), most Ainu burials fall within the arc of the sun’s rise and set between the summer and winter solstices. Yet many of the large kofun south of the Yamato River do not align with this pattern. While certain directional consistencies suggest regional traditions of belief, many kofun face the opposite way, making it nearly impossible to determine a definitive meaning behind their orientations.

It is also risky to interpret these orientations purely through the lens of ancient Japanese views of life and death. The orientations may have been deeply tied to the geography of the Osaka plain and to the differing beliefs and origins of each region. Some kofun may have been aligned toward the direction of a group’s homeland. It must also be considered that people of that age may not have possessed a philosophy of life and death in the sense we imagine at all. Furthermore, the Osaka plain today has been entirely leveled and urbanized, making it extremely difficult to reconstruct the ancient landscape in historical analysis.

One particularly troublesome factor when studying the ancient Osaka plain is the Yamato River. Today, the Yamato River flows westward from Nara through Kashiwara City, between Sakai and Osaka City, and into Osaka Bay, but this is actually a rerouted course from the Edo period. Historically, the Yamato River was a notorious flood-prone river, often wreaking havoc with large-scale floods. In 1704, during just eight months from February to October, the Tokugawa shogunate carried out massive engineering works, cutting off its northwestward course that had once joined the Yodo River near Osaka Castle, and instead redirecting the river westward.

Thus, in only eight months, the Yamato River was separated from the Yodo River and began flowing directly into Osaka Bay. The current course of the Yamato River between Sakai and Kashiwara has a history of barely 300 years, making the landscape entirely different from that of the kofun era.

This fact makes it all the more significant that the Mozu and Furuichi kofun clusters, now UNESCO World Heritage sites, are concentrated south of the Yamato River. In other words, the river as we know it today did not exist when the kofun were built. Yet, considering their placement, it appears that the people of that age already had a sophisticated understanding of where to build these monumental structures within the Osaka plain. Indeed, there are no prominent kofun north of the Yamato River, raising the question of why this is so.

Even with the advanced engineering of the Edo period, the rerouted river underscores that the immense kofun concentrated in the southern region were deliberately placed with comprehensive awareness of geography. Daisenryo Kofun and other colossal burial mounds feature moats with water channels intricately arranged, often in triple layers. Where exactly did this water come from? This remains an unsolved mystery. Perhaps by viewing history anew—stepping outside the linear timeline—we may begin to uncover what has long been overlooked.