JAPANESE underwater archaeologists have found evidence of the great invasion fleet sent by Kublai Khan in the 13th century, which tradition says was destroyed by a kamikaze or “divine wind” sent by the Emperor’s deified ancestors to save Japan from its enemies. Only a small proportion of the force was Mongol, the evidence shows: the majority was drawn from conquered China, and used advanced weaponry including shrapnel-filled projectile bombs.

The discovery, by Kenzo Hayashida of the Kyushu Okinawa Society for Underwater Archaeology, follows years of patient searching of the sea bottom off the north coast of Kyushu, the southernmost of Japan’s four main islands. The site, in Imari Bay, was initially found by fishermen, whose nets brought up artefacts including the personal seal of a Mongol commander, inscribed in both Chinese and the Phagspa script used to write the Mongolian language after the descendants of Genghis Khan conquered China and needed to administer their empire.

Sonar surveys and diving over the past 20 years have brought up iron swords, stone catapult balls, spearheads and stone anchor weights, James Delgado, of the Vancouver Maritime Museum, reports in the American journal Archaeology this month. The granite used for the anchor on the newly discovered shipwreck shows that the ship came from Fujian on the south China coast, one of the marshalling points for the fleet that attacked Japan in 1281.

More than 4,000 ships and thousands of troops were furnished by the defeated Sung Dynasty, according to Chinese records, and Kublai Khan’s Korean allies were ordered to build 900 more and to provide 10,000 soldiers. An earlier invasion attempt, in 1274, reportedly involved 23,000 men shipped across the Strait of Tsushima from Korea; they established a bridgehead and looted the port of Hakata (modern Fukuoka), but withdrew with the loss of numerous ships and more than half the army, according to some sources.

When Kublai invaded again in 1281, the Japanese were ready and had fortified the coast. The Korean section of the Mongol fleet attacked without waiting for the much larger Chinese force, and while they pondered how to attack the Japanese defensive walls, were in turn raided by small craft carrying samurai warriors, and by fireships.

After the main Chinese fleet arrived, a sudden storm, which the Japanese hailed as a heaven-sent kamikaze, mauled the anchored ships, drowning nearly all the 100,000 troops on board. At the entrance to Imari Bay “a person could walk across from one point of land to another on a mass of wreckage”.

It is one of these ships that the archaeologists have been investigating. Dr Delgado reports “bright red leather armour fragments, an intact Mongol helmet, a cluster of iron arrow tips, and a round ceramic object, a tetsuhau or bomb”. Such grenades were pottery spheres filled with gunpowder, and although their use is portrayed on scrolls depicting Kublai’s invasion, the historian Thomas Conlan has recently suggested in his book In Little Need of Divine Intervention that these were later interpolations.

“His suggestion that the exploding bomb is an anachronism has now been demolished by solid archaeological evidence”, Dr Delgado says. The six tetsuhau so far recovered “are the world’s earliest known exploding projectiles and the earliest direct archaeological evidence of seagoing ordnance”. X-rays of one bomb show that it was filled with pieces of iron shrapnel as well as gunpowder.

In spite of the find’s importance, excavations were hurried because a fish farm was due to be constructed in Imari Bay, and only a fraction of the necessary conservation has yet been funded; Japan is still in economic recession. Although Kenzo Hayashida and Thomas Conlan agree that hundreds rather than thousands of wrecks lie in the bay, the find is “one of the greatest underwater archaeological discoveries of our time, proving critical new information about Asian seafaring and military technology”, according to Dr Delgado. The area also has patriotic resonance: out in the Strait of Tsushima, the tsarist fleet was obliterated in 1905, in a naval battle that established Japan as a major modern power.