An Antiquity study counts 500 La Tène iron bars in a tonne of metal dredged from the Danube

On average the complete bars were 28.5 cm long and weighed around 2 kg.

The assemblage comprises at least 500 iron semi-products weighing more than 1 metric ton.

The collection dates to the third to second centuries B.C.

From hundreds of iron fragments recovered from the gravel heaps, the researchers identified 193 complete iron bars and 307 partial bars, for a total of at least 500 bars.

The Deinham mass find is the most abundant assemblage of Iron Age bars currently known in Europe, and the largest mass find of prehistoric iron bars in Europe.

Trebsche said the iron semi-products found at Deinham weigh about 1 metric ton, which is enough to forge approximately 500 swords with an average weight of 1.5 to 2 kg.

Trebsche said the Deinham iron could alternatively have produced up to 8,000 iron nails.

"Even if we take material losses during manufacture into account, the iron from Deinham would have been sufficient to equip a large group of warriors with weapons," Trebsche said.

"The possibility of ritual deposition cannot be excluded," the researchers wrote, but "most studies on river finds have concluded that iron bars are losses incurred during transport."

Similar iron bars have been found at Manching, Germany, about 200 km away, so the team wrote that it is plausible the Deinham bars were forged in Bavaria.

Researchers are currently analyzing traces of elements in the bars, which may help clarify their geographical origin.

"It is incredible that such a large quantity of iron was transported at once and was available in a single exchange transaction in the Late Iron Age," Trebsche said.

"It is nothing short of a minor miracle that the largest mass find of prehistoric iron bars in Europe did not end up at scrap dealers, but was reported to the Austrian monument authority," Trebsche said.

The exact scale of Iron Age iron trade is poorly understood because iron semi-products often escape the attention of finders and are difficult to date precisely.

The paper is titled "A sunken ship cargo of 500 bipyramidal bars? Implications for La Tene period iron production and trade along the Danube."

Manching is located near a large iron mining area, and several iron-production sites have been found in Bavaria near oppida, the large fortified towns of Iron Age Europe.

The Deinham bars were not finished products; the double-pointed, semifinished bars were rough-shaped intermediate goods that a blacksmith would eventually have forged into other objects.

The iron was likely lost in a shipwreck while being transported down the Danube for trade, which the authors say points to large-scale, well-organized inland shipping by later prehistoric Central European societies; the primary destination of the bars remains unknown.

A La Tene hoard at the Bavarian oppidum of Manching held 443 coins totalling 3.7 kg of gold, and the authors say these coins could plausibly have been equivalent in value to a metric ton of iron.

Because the iron was found during gravel extraction with a large bucket excavator, the original context of the find was destroyed, erasing archaeologists' opportunity to work out how the metal ended up in the Danube.

At 2 kg per sword a metric ton of iron yields exactly 500 swords, and at 1.5 kg per sword it yields about 667, so the 500-sword estimate uses the heavy end of the stated sword range with no forging loss.

Up to 8,000 nails from about 1 metric ton of iron implies roughly 125 g of iron per nail.

The 193 complete bars account for about 386 kg at the stated 2 kg mean, leaving roughly 614 kg across the 307 partial bars, or about 2.0 kg each.

Treating 3.7 kg of gold as equivalent to a metric ton of iron implies about 270 kg of iron per kilogram of gold.

The 193 complete bars are about 39% of the 500-bar total.