Distinctly shaped ancient burial grounds known as “kofun” are found all across Japan. Almost identical to the shape of a keyhole, the structures are ancient burial mounds built thousands of years ago. Kofun range from several metres to over 400m long, with the largest – the Daisen Kofun – having been attributed to Emperor Nintoku, in Sakai City, Osaka.
Thought to have been erected over a period of nearly 20 years in the mid fifth century – the Kofun Period – the Daisen Kofun was recently awarded Unesco World Heritage status.
More generally, across the country, kofun have been dated to between the early third and seventh centuries, nearly all of them stumping archaeologists and researchers as to what truly lies deep beneath them.
The Daisen Kofun was once home to over 100 tombs, but has shrunk in size over the centuries.
It has drawn particular attention because of its size, as a researcher, Hiroshi Kaibe, explained to the BBC last year: “In terms of surface area, it is actually bigger than the Egyptian pyramids.
“And, in Osaka, there is the Koshien baseball ground, it would take up 12 of those.”
Talking through the mystery of the mega-structure, he said: “When the Nintoku Emperor’s Tomb – Daisen Kofun – was first built by past methods, without machines, it must’ve taken 2,000 workers and a continuous 15 years and eight months to finish.”
Meanwhile, Izumi Tachibana, a curator and researcher at the Sakai City Museum, told of how a large proportion of the ancient sites had not yet been excavated.
Thus, little is known about the tombs beyond their superficial structure and historical accounts.
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“However, this was written a few hundred years after the kofun was actually made.
“Therefore, even today it is still discussed who was actually buried there.”
Many have disregarded the tomb as simply a shrine made of dirt and shrubbery.
Yet, as one researcher said: “The kofun had three levels; then clay figures called Haniwas were placed all around the flat surface.
“The Haniwas were also placed in the lower stages, so the structure was surrounded by three levels of Haniwas.”
It is estimated that as many as 29,000 Haniwas figures once lined the levels of the Daisen Kofun, carved in the shape of houses, armour, helmets, as well as giant stone crocks.
Researchers have made a guess as to why the kofuns all around Japan are based on what looks like a keyhole.
Noriyuki Shirakami, an historian, said: “The strongest interpretation seems to be that it was originally a circular tomb, then around they made hollow ditches.
“So, the outside and inside were separated, but one part of the ditches had to connect with the land so that people could get inside, to the circular tomb.
“So, the initial purpose was access, but then forgetting about the access, the shape stuck and became a Zenpokofuen – the keyhole shaped Kofun – that’s the theory.”
Despite their appeal, kofuns are strictly off-limits to visitors, with researchers and archaeologists also largely unable to access them.
The lack of access has spurred many theories, one suggesting they contain “embarrassing” secrets about Japan’s imperial history.
Those who work closely with the kofuns, however, say the reason for the restricted access is for preservation.
They say the tombs are “sacred” to Japan and must be respected, without being meticulously examined, taken apart and damaged.
Ms Tachibana said: “In the future we might have more advanced technology which wouldn’t be as destructive to the Kofun, yet could be more effective in finding new information.
“If we investigate now, such information could get lost forever.”