Daisen Kofun: Japan's Giant Keyhole Tomb of Emperor Nintoku

The Daisen Kofun, officially designated the tomb of Emperor Nintoku, is the largest keyhole-shaped burial mound in Japan. Built in the mid-5th century AD in what is now Sakai, Osaka Prefecture, it is about 486 metres long and surrounded by moats. It is one of the greatest monuments of the Kofun period, when the rulers of the Yamato state expressed their power by raising enormous earthen tombs, and since 2019 it has been part of a UNESCO World Heritage Site.

The Kofun period

The Kofun period (roughly AD 250 to 538 or later) is named after the kofun, or “old mounds”, built for the elite across much of Japan. It followed the Yayoi period, when rice farming, metalworking and walled villages such as Yoshinogari spread through the islands.

The most distinctive tombs were keyhole-shaped (zenpō-kōen-fun), with a square front joined to a round rear mound where the burial chamber was usually placed. The largest of them are concentrated in the Nara basin and the Osaka plain, the heartland of the Yamato kings, whose power gradually extended over much of western and central Japan.

These rulers were in contact with the Korean kingdoms and with China. Chinese histories record embassies from the “Five Kings of Wa” to the Liu Song court in southern China between 421 and 478, and Korean sources such as the Gwanggaeto Stele mention Wa forces fighting on the Korean Peninsula around 400.

Size and design

The Daisen Kofun lies on a terrace near the old shoreline of Osaka Bay, and its builders may have intended it to be seen from the sea. Its key dimensions, as usually cited, are:

The mound was built in three terraced tiers. Its slopes were originally covered in stones and lined with rows of haniwa, hollow clay cylinders and figures placed around tombs. Today the mound is covered in dense forest that has grown over the centuries.

Measured by ground area, the Daisen Kofun is often described as one of the largest tombs in the world, alongside the Great Pyramid of Giza and the mausoleum of the First Emperor of Qin, although each is enormous in a different way.

Who built it?

The tomb is designated by the Imperial Household Agency as the Mozu no Mimihara no Naka no Misasagi, the tomb of Emperor Nintoku. In the 8th-century chronicles Kojiki (712) and Nihon Shoki (720), Nintoku is the 16th emperor. Tradition places his reign from 313 to 399, dates that historians do not accept as accurate.

The Nihon Shoki portrays Nintoku as a virtuous ruler. A famous story tells how he noticed that no smoke rose from the people’s kitchens, realised they were too poor to cook, and suspended taxes and labour duties for three years. Such stories reflect later ideals of kingship rather than verified events.

Archaeologists date the mound by the style of its haniwa and pottery to the mid-5th century, later than Nintoku’s traditional reign. Many therefore use the neutral name Daisen (or Daisenryō) Kofun, after the local place name. The person buried there was certainly a great king of Yamato, but which one is uncertain; some scholars have linked the mound to one of the Five Kings of Wa named in Chinese records.

Labour and construction

No records describe how the tomb was built. In 1985 the Japanese construction company Obayashi estimated that building it with ancient methods would have required about 2,000 workers a day for nearly 16 years, a total of around 6.8 million person-days. Such figures are modern calculations, but they convey the scale of the effort.

Workers dug the moats and used the excavated soil to raise the mound, compacting it in layers. Specialist potters produced the haniwa in large numbers, and stone for the surface was brought in from elsewhere. The project implies a ruler able to command labour from a wide area, which historians see as evidence of a strong, centralising Yamato state.

Discoveries and access

Because it is treated as an imperial tomb, the Daisen Kofun has never been systematically excavated. In 1872 a landslip on the front part of the mound exposed a stone burial chamber with a long stone coffin and gilt-bronze armour. Drawings were made, and the chamber was covered again. A bronze mirror and a sword fitting in the Museum of Fine Arts, Boston, are said to come from the tomb, but this is not certain.

In 2018 the Imperial Household Agency and Sakai city carried out a limited joint excavation of the inner bank, the first such cooperation at the site. It confirmed a paved stone surface and rows of haniwa, and surveys suggested that the original mound may have been larger than the official length.

Myths and facts

• “It is certainly Nintoku’s tomb.” The attribution comes from later tradition and official designation; the archaeological date does not match Nintoku’s traditional reign, and the occupant has not been identified.
• “Its contents are fully known.” Only the 1872 glimpse of the front chamber is documented; the main burial in the round rear mound has never been opened or investigated.
• “It was always a forest.” The trees grew after the tomb was completed; originally the slopes were faced with stones and ringed with haniwa, giving it a bare, terraced appearance.
• “It is the world’s biggest tomb.” It is among the largest by ground area, but comparisons with the pyramids or the Qin mausoleum depend on whether one measures area, volume or height.

Legacy

In July 2019 the Daisen Kofun was inscribed on the UNESCO World Heritage List as part of the Mozu-Furuichi Kofun Group, 49 mounded tombs in Sakai, Habikino and Fujiidera that illustrate the Kofun period. Visitors can walk the roughly 2.8-kilometre path around the outer moat and view the mound from an observation lobby in Sakai City Hall.

The tomb remains a sacred site for the imperial household, and debates continue over whether more research access should be allowed. For historians it is the clearest physical evidence of the power concentrated in the hands of the Yamato kings, ancestors of the later Japanese imperial line, well before the official introduction of Buddhism in the 6th century and the wider use of writing that followed.

Timeline

• c. 250: Large keyhole-shaped tombs begin to appear, marking the start of the Kofun period.
• c. 400: Wa forces fight in Korea, according to the Gwanggaeto Stele.
• 421–478: The Five Kings of Wa send embassies to the Liu Song court in China.
• Mid-5th century: The Daisen Kofun is built.
• 712 and 720: The Kojiki and Nihon Shoki record Nintoku as the 16th emperor.
• 1872: A landslip exposes a stone chamber on the front of the mound.
• 2018: First joint excavation by the Imperial Household Agency and Sakai city.
• 2019: Inscribed on the UNESCO World Heritage List.

Why it matters

The Daisen Kofun is the most striking monument of early Japanese state formation. Its scale shows how much labour and wealth the Yamato kings could command in the 5th century, and the questions over who lies inside it reflect the gap between Japan’s early chronicles and the archaeological record.