Notre Dame Cathedral, a UNESCO World Heritage Site, is the most visited monument in Europe, attracting 50 million tourists every year.

The beloved cathedral that lies on the lle de la Cite in the heart of Paris experienced a catastrophe last April 15 when a massive fire engulfed the wood roof, spire, and parts of the interior.

For the next 15 hours, Parisians stood on every side of the church, staring straight ahead, in disbelief and disempowerment as 400 firefighters battled the nearly inextinguishable blaze.

The most heartbreaking moment came when the wooden fleche, the spire, came crashing down.

Read more: Restoration of Europe's most-visited monument to take time