It took near destruction and a massive restoration project which is still in progress for a team of archaeologists to discover the iron reinforcements.

The construction of the famous cathedral in the heart of the French capital began in 1160 and was not completed until almost a century later.

It was the tallest building of its time, with vaults reaching up to 32 metres, according to a study published in the journal PLOS ONE on Wednesday.

Maxime L’Heritier, an archaeologist at University Paris 8 and the study’s lead author, told AFP that some elements of the building’s construction remained unknown, even after all these centuries.

It was not clear how the builders “dared — and succeeded — in putting up such thin walls to such a height,” he said.

Lacking much documentation from more than 900 years ago, “only the monument can speak” about its construction, he added.

The blaze on April 15, 2019, exposed  iron staples used to hold the cathedral’s stone blocks together.

Some appeared in the frame of the building, others fell smouldering to the ground in the heat of the blaze.

READ MORE: Paris’ Notre-Dame ‘will reopen by end of 2024’

‘Much more dynamic ‘

The cathedral could be riddled with more than a thousand iron staples, the study said.

There are staples of varying sizes, ranging from 25 to 50 centimetres long, some weighing up to a few kilos.

They were found in many different parts of the cathedral, including in the walls of the nave, the choir tribunes and in parts of the cornice.

“This is the first truly massive use of iron in a Gothic cathedral, in very specific places,” L’Heritier said.

Iron staples have been used in construction since Antiquity, including in Rome’s Colosseum and Greek temples.

But in those cases they were simply used to keep large stone blocks secure on the lower floors.

Notre-Dame has a “much more dynamic conception of architecture,” L’Heritier said.From the very beginning, the builders used the iron staples to make the cathedral’s stands in the early 1160s. Their successors continued their innovative use on the upper parts of the walls over the next 50 to 60 years.

Iron would go on to be used in this way in numerous cathedrals across France.

More than 200 scientists are working on restoring Notre-Dame, whose iconic spire is expected to back in place by the end of this  year.

The reconstruction project is on track to be completed by the end of next year, France’s culture ministry told AFP last month.

This means the tourist landmark, which previously saw 12 million annual visitors, will not be open when Paris hosts the Olympic Games in July and August 2024.