France, March 17 -- The fire that engulfed Notre-Dame Cathedral in central Paris four years ago exposed a long-hidden secret about the Paris landmark: it was the first Gothic cathedral in which iron staples were used as reinforcements during the construction of what was the tallest building of its time.

It took the near destruction and the ongoing restoration project to allow 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.

In the absence of detailed plans and documentation, architects have long been mystified as to how their medieval counterparts managed to build such thin walls to ...