The famed Notre Dame cathedral in Paris was ablaze Monday as shocked onlookers watched in heartbreak from nearby bridges.

Footage showed flames and smoke flowing from the historic structure, and its spire was engulfed before collapsing.

Construction began on the Gothic cathedral in 1163, and it took over 100 years to complete. Twelve million people stop by each year; it is the most-visited monument in France.

The church's site is full of symbolic decorations, colorful glass, art and gargoyles. The cause of the fire was not immediately known, but officials said late Monday that the church’s main structure had been saved after firefighters prevented the flames from spreading to the northern belfry.

Here are five things to know about the world-renowned structure:

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Why was Notre Dame cathedral built?

Maurice de Sully, who became bishop of Paris in 1160, wanted to reconstruct the site where Notre Dame sits, which had been a Romanesque church and pagan temple.

Paris was becoming a political, economic and intellectual center in Europe around that time, and de Sully wanted to mark its progress, Notre Dame's website says.

The cathedral's towers are more than 220 feet tall and the building is more than 400 feet long, according to Notre Dame's website. It's almost 52,000 square feet.

It was looted during the French Revolution

The cathedral was looted and damaged during the French Revolution, which was in part fueled by resentment against the Catholic Church and the rise of Enlightenment thinking.

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According to the Napoleon Foundation, the cathedral was renamed "Temple to the Goddess Reason" and at one point served as a storage place for wine.

When France and the Catholic Church reconciled in 1801, the cathedral was returned to the church. Napoleon later used it during his coronation, according to the foundation.

The Hunchback of Notre Dame

Victor Hugo's novel "The Hunchback of Notre Dame," published in 1831, centers on Quasimodo, the cathedral's bell-ringer, and Esmeralda, a kind and beautiful street dancer.

The novel has been adapted into a number of films, including the popular animated Disney version "The Hunchback of Notre Dame" in 1996.

It was home to relics tied to the Passion of Jesus

The cathedral's "most precious and most revered object" is the Crown of Thorns, according to France's tourism website. Before Jesus was crucified, Roman soldiers mocked him and stuck a crown made of thorns on his head. However, around the world, others claim to have portions of the crown or single thorns.

Other important relics in the cathedral include a fragment of the True Cross, believed to be part of the cross on which Jesus was crucified, and one of the Holy Nails, believed to be used in the crucifixion.

In addition to religious relics, masterpiece artworks and sculptures, stained-glass windows and three organs are also housed at Notre Dame, its website says.

Salvage efforts had been underway to recover precious artifacts, and city prosecutors announced they were opening an investigation.

"A special mission has been launched to try to save all works of art that can be saved," Emmanuel Grégoire, first deputy to the mayor of Paris, said on French TV.

There's a bee hive on its roof – and it's not there by accident

Notre Dame's roof became home to a hive of bees in 2013, according to the cathedral's website.

The hive was placed as part of a larger effort to protect biodiversity and prevent bee die-off.

Contributing: John Bacon and David Oliver, USA TODAY; The Associated Press

Follow USA TODAY's Ryan Miller on Twitter @RyanW_Miller