The intriguing enigma of Japan's keyhole tombs

The construction of keyhole tombs began during the early Kofun period, around the late third century AD.

Archaeological evidence shows that the earliest examples appeared in the Nara Basin, which was a centre of political power in Japan at the time.

The period saw the rise of influential clan leaders who consolidated territories and established hierarchical societies.

As time progressed, the size and complexity of the tombs increased. The largest of all, the Daisenryo Kofun in Osaka Prefecture, is traditionally attributed to Emperor Nintoku and dates to the late fourth or early fifth century AD.

Many other grand tombs were constructed across the Kinai region and further afield in Kyushu and Honshu, which suggests the expanding influence of powerful rulers. 

Keyhole tombs continued to be constructed for several centuries. By the late sixth and early seventh centuries AD, their use declined as political authority shifted toward new centres and Buddhist burial practices gained popularity.

The gradual reduction in their size and frequency reflects significant changes in society, including the growth of a more centralised imperial government that adopted new cultural and religious traditions from the Asian mainland. 

Keyhole tombs served as burial mounds for elite individuals who held political and military power.

The effort required to construct such very large monuments indicates that those buried within had control over large numbers of labourers and resources.

Grave goods found in some tombs, such as bronze mirrors, iron weapons, and decorative items made of gold and jade, suggest that the occupants were part of a warrior aristocracy that valued military prestige and connections to continental trade networks. 

Other archaeological finds, including haniwa clay figurines that were placed on and around the tombs, indicate that funerary rituals were detailed public events.

Specifically, these figures depict warriors, attendants, animals, and structures. The tombs may have reinforced political authority by serving as conspicuous monuments to the power of a ruler and his clan.

Their construction likely involved the cooperation of subordinate groups. 

Builders excavated large moats around many tombs to contruct multiple tiers, which relied upon careful layering of soil to prevent collapse.

The largest tombs, such as the Daisenryo Kofun, covered areas of more than 45 hectares and must have required the coordinated work of thousands of labourers over several years. 

Grave chambers inside the mounds were often built with stone walls and ceilings, which formed protective chambers for the deceased and their grave goods.

Some chambers contained wooden coffins, while others had stone sarcophagi. The exact alignment of many tombs indicates that their builders used detailed surveying techniques, and the uniform keyhole shape across wide areas shows common cultural and political practices.

The sheer scale of the tombs demonstrates the ability of early Japanese rulers to mobilise large numbers of people and materials. 

Historians continue to debate the identity of many of the individuals buried in the keyhole tombs.

The Japanese Imperial Household Agency restricts excavation of the largest tombs because they are regarded as imperial mausoleums.

As a result, clear information about their occupants remains limited. Scholars rely on smaller tombs and nearby sites to understand their construction and use. 

The purpose of the distinctive keyhole shape also remains uncertain. Some theories propose that the design had symbolic meaning related to authority or ceremony, while others suggest practical reasons for accommodating ceremonial processions.

Because many of the largest tombs have never been excavated, questions remain about the specific identities, rituals, and beliefs of those who constructed them. 

Regardless, they continued to draw attention from researchers who want to finally uncover their full story.