Notre Dame holds some of the most precious Christian relics, including the crown of thorns and a piece of the holy cross.
In the 13th century, French king Louis IX (Saint-Louis) set about buying as many Christian relics as he could.
It's estimated he spent roughly 50% of the French budget on the purchases - which included the crown of thorns, a piece of the holy cross, a nail, the holy lance and the holy sponge - all immensely valuable in the Christian faith as they are intimately linked to the crucifixion of Christ.
He built the neighbouring Sainte Chapelle to preserve them.
The Biblical story goes that a week prior to his crucifixion, Jesus entered Jerusalem, expelled merchants from its temple and shared a supper with his apostles.
Betrayed by Judah, he was flogged, a crown of thorns mockingly placed over his head, before being nailed to a cross - a common punishment for criminals at the time.
A sponge was soaked in vinegar and given to him to drink.
To check whether he had died, a Roman soldier pierced his body with a lance, drawing blood and water.
When the Sainte Chapelle was looted during the French Revolution, the relics were transferred to Notre Dame.
They joined the existing Notre Dame relics - a white tunic belonging to Saint-Louis, part of his jaw, rib and teeth, as well as bone and hair from the Parisian saints Saint Denis and Sainte Genevieve.
Before the 2019 fire, these relics were shown to the public once a month under the supervision of the Knights of the Holy Sepulchre, a special military order which is under the sovereignty of the pope.