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Fire ravaged the 850-year-old church for some 12 hours before it was declared extinguished.

Nearly 400 firefighters battled the flames that altered the city's skyline.

The blaze collapsed the cathedral's spire and spread to one of its rectangular towers, as throngs of spectators watched in stunned horror from nearby buildings, bridges and streets.

So, why did the Notre Dame burn so fast?

Well, its medieval-roof framework consisted predominantly of huge pieces of timber.

The supports were made up of large pieces of wood and were built from an estimated 13,000 oak trees.

The original parts of the frame would have been made from trees that first sprouted in the eighth or ninth century.

Think all the way back before Napoleon, Marie Antoinette, Louis XVI and Joan of Arc, all the way to the Vikings, Charlemagne, and the decline of the Mayan civilization.

Gun powder had only just been invented when these oak trees came to life.

Over the years, as the church fell into disrepair, crumbling stone was replaced with even more wood, increasing the fire risk.

Notre Dame's website even warned that fire is not impossible.

As Notre Dame burned, the outpouring of grief around the world reflected the church's sweeping place in history.

One of Europe's most iconic structures, the cathedral houses a collection spanning nearly 1,000 years, containing French Gothic, Renaissance and Baroque elements.

In fact, 400 years after its birth, French poet Victor Hugo wrote The Hunchback of Notre Dame.

At the time, his novel made people more aware of the importance of the deteriorating Gothic architecture, resulting in restoration while placing the Cathedral in the minds and hearts of people for hundreds of years to come.

French President Emmanuel Macron vowed to reconstruct the historic building, even as the fire still burned.