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It was the sword of choice for the discerning Viking - superstrong, and almost unbeatable in battle.

Yet mystery surrounds a small number of Viking swords researchers have uncovered.

They are all inscribed with a single word - 'Ulfberht', which experts believe may reveal their maker.

a single word - 'Ulfberht' - on the blade of a Viking sword. Experts believe a German monastry may have been responsible for the product of the superstrong weapons.

About 170 Ulfberhts have been found, dating from 800 to 1,000 A.D. They are made of metal so pure it baffled archaeologists, who thought the technology to forge such metal was not invented for another 800 or more years, during the Industrial Revolution.

According to Ancient Origins, researchers are now closing in on the mysterious maker.

'New research brings us closer to the source of the swords, to the kiln in which these legendary weapons were forged,' it claims.

About 170 Ulfberhts have been found, dating from 800 to 1,000 A.D. 

They are made of metal so pure it baffled archaeologists, who thought the technology to forge such metal was not invented for another 800 or more years, during the Industrial Revolution.

Alan Williams of the Wallace Collection in London has studied the blades, and believes the maker is unique.

'It's much like putting the 'Apple' name on a computer,' he said.

They were extremely rare and valuable, and would have been prized possessions of the most elite Vikings. 

Robert Lehmann, a chemist at the Institute for Inorganic Chemistry at the University of Hannover, studied an Ulfberht sword found in 2012 on a pile of gravel excavated from the Weser River, which flows through Lower Saxony in northwestern Germany. 

This sword's blade has a high manganese content, which signalled to Lehmann that it did not come from the East. 

The guard was made of iron with a high arsenic content, which suggests a European deposit. 

He traced the lead to a site in the Taunus region, just north of Frankfurt, Germany - where he believes it may have been made. 

While some monasteries in the Taunus region are known to have produced weapons at that time, the name of Ulfberht has not been found in their records. 

Experts have even tried to recreate the construction methods of the swords to work out how they were made.

The manufacturing process used has also baffled researchers.

In the process of forging iron, the ore must be heated to 3,000 degrees Fahrenheit to liquify, allowing the blacksmith to remove the impurities, known as 'slag'

Carbon is also mixed in to make the brittle iron stronger. 

Medieval technology did not allow iron to be heated to such a high temperature, so slag was removed by pounding it out, a far less effective method.

The Ulfberht, however, has almost no slag, and it has a carbon content three times that of other metals from the time. 

It was made of a metal called 'crucible steel.'

It was thought that the furnaces invented during the industrial revolution were the first tools for heating iron to this extent. 

Modern blacksmith Richard Furrer of Wisconsin spoke to NOVA about the difficulties of making such a sword.

'To do it right, it is the most complicated thing I know how to make,' he said.