From the perspective of those on the ground, one of the world's largest graves appears to be an expansive, natural respite nestled in the bustling urban core of Japan's Sakai City. When viewed from above, however, the massive forest becomes a green keyhole surrounded by three moats in the shape of a doorway.

This is Japan's Daisen Kofun, an ancient burial mound shrouded in mystery, massive in scale, and surprisingly little-known to the rest of the world.

A side view of the massive Daisen Kofun burial mount bordered with Sakai City in Japan.
A side view of the massive Daisen Kofun burial mount bordered with Sakai City in Japan. (Photo: 無料動画素材TV/YouTube Screenshot)

Much like the ancient Egyptians built pyramids to honor deceased royalty, the Japanese interred their emperors and other notable figures in megalithic structures known as kofun or tumuli. Between the early 3rd century and the early 7th century A.D., during Japan's Kofun period, an estimated 200,000 keyhole-shaped tombs were constructed throughout the Japanese archipelago.

The Daisen Kofun, believed to be at least 1,600 years old, is the biggest tumulus and one of the three largest tombs in the world, sharing court with both the mausoleum of the First Qin Emperor in China and the Great Pyramid of Khufu in Egypt.

While the entire moat-ringed complex spans 110 acres, the tumulus itself measures an impressive 1,600 feet long by 980 feet wide. It is part of a cluster of smaller tumuli, called the "Mozu kofungun," which are currently under consideration for inclusion on the UNESCO World Heritage List.

The entrance to the Daisen Kofun in Sakai City, Japan.
The entrance to the Daisen Kofun in Sakai City. (Photo: Jean-Pierre Dalbéra/Flickr)

Unlike other historical wonders, however, you can't actually visit this marvel of the ancient world. In fact, since the late 19th century, the Daisen Kofun has been expressly off-limits to tourists, archaeologists and even royalty. Reportedly, no one has crossed the inner-moat and visited the island since 1872, when a typhoon damaged the lower part of the keyhole shape. The Imperial Household Agency, which manages the Daisen Kofun and others like it throughout Japan, has prohibited human contact on the grounds that "tranquility and dignity" should be maintained at the sacred site.

As such, the agency has been content to let the tomb revert to a natural state, with the grounds covered in a thick forest of native trees and the surrounding moats providing sanctuary for fish and waterbirds. Today, tourists interested in viewing the site can do so only from either a platform across the second moat or by walking the nearly-two mile path around the tomb's outer dike.

The tomb as viewed through Google Earth.
The tomb as viewed through Google Earth. (Photo: Google Earth)

Because of the strict nature of access to Daisen Kofun, it's not exactly clear who is buried under the forested canopy of what's considered the world's largest grave by area. Agency officials believe the site was created for the early 5th century Emperor Nintoku, though it has never been confirmed if his body or any other members of his royal family are interred on the island.

While appeals have been made over the last several decades to allow unrestricted access to the tumulus for research, all have been rebuffed.

A 3D illustration of the layout of a kofun tomb stripped of its vegetation.
A 3D illustration of the layout of a kofun tomb stripped of its vegetation. (Photo: 名古屋太郎/Wikimedia)

Based on other keyhole mounds excavated around Japan, the upper circular part of the structure is where the bodies were entombed, while the lower rectangular area was used for mortuary rites. A 1995 article in the Independent reported that there may be as many as "26,000 tons of stone slabs" buried beneath the kofun, with "swords, jewels, crowns, statues, and the coffined remains of the great god- emperor himself" sealed inside.

Of course, as far as anyone knows, no one has explored the circular region of the tumulus in more than 16 centuries. The biggest question of whether or not the treasures of the emperor lie undisturbed remains unanswered.

An aerial view of the various keyhole tombs that dot the landscape of Sakai City, Japan.
An aerial view of the various keyhole tombs that dot the landscape of Sakai City. (Photo: Bing Maps)

There are signs of progress, however, at least when it comes to discovering a bit more about Daisen Kofun.

Late in October, the Imperial Household Agency began conducting a joint excavation survey with the Sakai municipal government on one of the dikes surrounding the tomb. While the agency insists this latest action is meant to help determine future conservation efforts for the site, archaeologists are nonetheless intrigued by what they may discover. A previous survey conducted in 1973 uncovered clay figures made for ritual use.

As yet, however, the main burial mound remains untouched — a frustrating and mysterious link to Japan's past that some believe deserves critical attention.

"The agency should change its stance and allow large-scale academic research to uncover facts about the situation surrounding the construction of the kofun including the identities of the buried people so that reliable and confirmed information about this historical site can be provided internationally," declared one Japanese editorial.

"To uncover the historical value of these sites for future generations, it is vital to take advantage of the latest archaeological and historical knowledge in research and make the findings widely available to the public through inspection tours and exhibitions," they added.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.