The sheer scale of Japan's largest kofun defies expectation. The Daisen Kofun in Sakai, attributed to Emperor Nintoku, stretches approximately 486 meters in length and over 300 meters in width, surrounded by three concentric moats. Its footprint covers more ground than the Great Pyramid of Giza. Construction is estimated to have required some 2,000 workers laboring daily for nearly 16 years. When the Mozu-Furuichi Kofun Group -- the cluster of burial mounds around Osaka that includes the Daisen Kofun -- was inscribed as a UNESCO World Heritage Site in 2019, it drew global attention to a form of monumental architecture that had long been overshadowed by Egypt and Mesoamerica. Approximately 160,000 kofun of varying sizes were built across the Japanese archipelago during this era, though only a fraction were imperial. The distinctive keyhole shape, called zempo koen fun, combined a circular rear mound with a rectangular front platform, creating an unmistakable silhouette visible only from above.

The Imperial Household Agency's grip on these sites is absolute. As the administrative body responsible for the affairs of the imperial family, the Agency designates 188 burial mounds throughout Japan as official imperial graves, and each one is fenced, gated, and guarded. Researchers have lobbied for decades to be allowed inside, arguing that modern non-invasive techniques could reveal crucial information about early Japanese state formation without disturbing the burials. The Agency's position is unwavering: these are not archaeological sites but active religious spaces where the spirits of emperors rest. Some scholars have questioned whether all 124 tomb assignments are historically accurate, since many were designated during the Meiji era based on limited evidence. The irony is profound -- the very sites that could answer fundamental questions about the origins of the Japanese state are locked behind gates that only imperial priests may enter.

The evolving forms of imperial tombs trace the arc of Japanese political and cultural history. Early legendary emperors received massive keyhole-shaped kofun, asserting raw power through sheer volume of earth and stone. By the seventh century, octagonal forms appeared, signaling a shift toward Chinese-influenced court culture and Buddhist ideas about the afterlife. The Asuka-period tombs of Nara Prefecture are markedly smaller and more refined, with elaborately cut stone burial chambers replacing the earlier earthen interiors. When Buddhism took firm hold, imperial burial customs shifted again: emperors of the Kamakura and Muromachi periods received modest memorial halls and pagoda-style monuments rather than earthen mounds. The Edo period saw a return to ceremonial formality, with many emperors interred beneath nine-tiered stone pagodas in Kyoto temple complexes. Finally, the modern emperors -- Meiji, Taisho, and Showa -- were buried in circular mounds atop square bases, a style called joendoho that consciously evoked the grandeur of the ancient kofun while incorporating contemporary design.

The registry of imperial tombs is, in essence, a physical map of the Japanese monarchy's 2,600-year mythological timeline laid across the landscape of western Japan. The earliest entries -- Emperors Jimmu through Kaika -- are considered legendary by historians, their assigned tombs likely dating to periods centuries removed from the lives they are supposed to memorialize. The historical record grows firmer from Emperor Ojin onward, and the keyhole-shaped kofun of the fifth century represent perhaps the clearest physical evidence of centralized power in ancient Japan. Notably, the list includes eight empresses -- women like Suiko, Kogyoku, and Meisho -- whose tombs confirm that female sovereignty was not anomalous but recurring. The geographic concentration of these sites in the Kinki region, particularly around Nara, Osaka, and Kyoto, reflects the political center of gravity for most of Japan's recorded history. Walking among them today, with their forested crowns rising above modern housing developments and convenience stores, feels like encountering time itself buried just beneath the surface.