But the most remarkable thing about Nintoku is his final resting place. In the town of Sakai, a few miles south of the gleaming hotels and skyscrapers of Osaka, it stands even today - a vast, keyhole-shaped burial mound, 500 yards long, surrounded by earthen ramparts and a water-filled moat. Its area is half that of a good-sized pyramid; 26,000 tons of stone slabs are believed to lie beneath the tumulus, covered by an immense volume of heaped earth and trees. Inside there may be swords, jewels, crowns, statues, and the coffined remains of the great god- emperor himself. On the other hand, there may be nothing there at all. Nobody knows because, officially at least, nobody has been allowed inside for 1600 years.
This is not for want of trying. "It's a fiasco, a scandal, unprecedented in a civilised country," says one academic. Throughout its history, Japanese archaeology has laboured under an extraordinary handicap: the tombs of the Japanese emperors, some of the most important historical sites in east Asia, of which Nintoku's is the biggest and most famous, have been placed entirely off-limits by the Japanese government. This summer, after years of lobbying, archaeologists were told by ministers that plans were afoot to lift the ban. But after a cabinet reshuffle they were once again put on hold. "For 50 years I have been studying this field," says Professor Hatsue Otsuka, of Meiji University, "and I feel as if I have just been paddling round the edges. I have never been allowed into the deep water."
There are more than 200,000 ancient burial mounds in Japan, most of them originating from the so-called Tumulus Era, between the 4th and 8th centuries. Few written records survive from this period, making them a crucial source of historical information. Thousands of lesser tombs, belonging to local lords and princelings, have been systematically excavated to reveal unique 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: 848 of them, including those of 123 dead emperors, from Jimmu (711-585 BC) right through to the modern mausoleum of Hirohito (1922-1989).
This is where the problems begin. To the archaeologists, the tombs are cultural properties like any other, a legitimate object of scientific study. But they are owned and administered by the Imperial Household Agency (IHA), the government department responsible for all matters relating to the emperor and his family. Even by the uptight standards of Japanese bureaucracy, the IHA is notorious for its conservatism. And according to the agency, the tombs are more than just historical relics; they are sacred religious sites, the sanctuaries of the spirits of the imperial ancestors. An annual 200m yen (pounds 1.25m) is spent on their upkeep; every year envoys offer Shinto prayers and gifts on behalf of the emperor.
This is a controversial enough position in itself. Emperor Hirohito famously renounced his divinity in 1946, and Japan's post-war constitution, drafted by the American occupation during the same period, unequivocally insists that "the state and its organs shall refrain from religious education or any other form of religious activity". 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. In 1978, when Professor Otsuka published an article calling for the opening up of the tombs, he was placed under police protection after receiving death threats from right-wing groups.
Many archaeologists suspect that there is more to the agency's reluctance than its stated desire to maintain the "quiet and dignity" of the tombs. It is almost certain, for instance, that several of the tombs are incorrectly attributed. As Nintoku's lively biography makes clear, the early emperors are as much legendary figures as historical personages, and excavation around the periphery of several tombs has thrown the IHA's dating of them into doubt. The artefacts found in lesser tumuli, moreover, display striking similarities with continental relics, additional proof that much early Japanese culture was imported from Korea. The idea that the early emperors were Korean princes - a race colonised by Japan until 1945, and still the object of prejudice and rivalry - may be more than the Imperial Household Agency can bear.