OF ALL the monuments of old Japan, there are none so mysterious as the burial mounds of the ancient emperors.

Thickly wooded, surrounded by murky moats, they lie scattered across the Japanese countryside, as central to its history as the pyramids are to ancient Egypt.

Beneath the heaped mounds may be rich treasures — gold-encrusted swords, jade jewels, elaborate crowns and figurines of men and animals sculpted out of clay. There may be sacred mirrors from China and Korea, exquisite wall paintings of tigers and dragons, and the coffined remains of the great god-emperors themselves — or there may be nothing at all.

Nobody knows, for the imperial kofun, as the tumuli are known, are as closed now as they were when they were sealed 17 centuries ago.

To the frustration and fury of Japanese archaeologists, the country’s Imperial Household Agency refuses to allow any inspection of the tombs, let alone excavation. To the agency, they are places of sacred Shinto worship, the dwelling place of the souls of the deified emperors.

But now the discovery of secret agency documents has reopened the kofun controversy and provided hope that eventually their hidden treasures may be exposed to the outside world. Archaeologists allege that the agency has known for many years that imperial tombs were wrongly identified but was ashamed to acknowledge its mistakes.

Academics who have challenged the ban have received death threats from right-wing ultra-nationalists.

Noboru Toike, a professor of imperial history at Tokyo’s Den-en Chofu University, said: “This is one of the most significant periods of our history, and the tombs are a treasure trove of information. They are the cultural property of the nation, and they have to be preserved and opened for the benefit of the public. It makes me angry.”

The controversy over the tombs raises sensitive questions about the origins of the Japanese people, and the present-day character of Japan’s Imperial Family.

There are more than 200,000 ancient burial mounds in Japan, most of them originating from the Tumulus era, from the 4th to 7th centuries, a period from which few written records survive. Thousands of lesser tombs, belonging to local lords and princelings, have been systematically excavated to reveal historical information about daily life under the rulers of Yamato, the precursor of medieval Japan.

But the biggest and most important of all are the imperial tombs: 896 of them, including those of 124 dead emperors, from Jimmu, the legendary founding father of the 7th century BC, right through to the modern mausoleum of Hirohito (1901-1989).

To archaeologists, the tombs are cultural properties like any other, a legitimate object of scientific study. To the notoriously conservative Imperial Household Agency, they are sacred religious sites. In religious terms, it is rather as if the prehistoric sites of Britain were placed off limits to scientists because of the objections of Druids.

Every year envoys offer Shinto prayers and gifts on behalf of the Emperor, which is controversial in itself. Emperor Hirohito famously renounced his divinity in 1946, and Japan’s postwar Constitution, drafted under the American occupation during the same period, unequivocally insists on the separation of religion and state. To older Japanese, the discreet continuance of the emperor cult is a sinister reminder of the militaristic “state Shinto” of the wartime period, espoused by a minority even today.

When Hatue Otsuka, a professor at Meiji University, published an article calling for the opening of the tombs in 1978, he was placed under police protection after receiving death threats.

One reason for the agency’s intransigence may lie in what the tombs could reveal about the origins of the emperors.

In the official version, they have existed in an unbroken line since Jimmu, the great-great-grandson of the Sun Goddess, Amaterasu Omikami. But historians suspect that there was much intermarriage with present-day Korea and China, and that the early rulers of Japan may even have come from continental Asia.

“At one time, Japanese regarded the people of Korea as lower than us,” Professor Toike said. “For such people, it is difficult to accept that the Imperial Family is descended from Korea.”

But there may be a less elaborate explanation for the agency’s reluctance — simple embarrassment.

For years, historians have suspected that many of the so-called imperial tombs were mistakenly identified in the 19th century. This month, under Japan’s Freedom of Information Act, Professor Toike obtained agency documents showing that the agency itself was aware of this 50 years ago. Professor Toike said: “This is nothing less than proof of the (agency’s) lies.”

With stubborn ingenuity, the agency refuses to be embarrassed even by this: one archaeologist was told that, even if the tombs were wrongly attributed, the sprits of the emperors assigned to them would have moved there anyway, drawn by centuries of mistaken, but sincere, worship.

EMPIRE STATE

• Emperor Jimmu (660-585BC) was the mythical first emperor of Japan, said to be descended from the Shinto goddess Amaterasu.
  
• Emperor Ojin (AD270-310), the first emperor thought to have truely existed, is now honoured as the guardian of warriors.
  
• Emperor Meiji (1852-1912) introduced a Western-style parliament and consitution and established Japan as a world power in wars against China (1894-95) and Russia (1904-05).
  
• Emperor Hirohito (1901-1989) dissolved the link between Shinto religion and the Imperial family.

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