Prior to the fire, Notre Dame held some of the most precious Christian relics, including a piece of the Holy Cross and the entire Crown of Thorns.
In the 13th century, Louis IX, the future Saint Louis, set about buying as many relics as he could.
He spent around half of the national budget on the purchases - which also included the Holy Lance and the Holy Sponge.
The Sainte Chapelle, near Notre Dame, was built to contain them.
When the Sainte Chapelle was looted during the French Revolution, the relics were transferred to Notre Dame.
They joined its existing relics - a white tunic belonging to Saint Louis, as well as part of his jaw, rib and teeth, and some bone and hair from Saint Denis and Saint Genevieve.
Before the fire, the relics were shown to the public once a month, under the supervision of the Knights of the Holy Sepulchre, a special military order under the sovereignty of the pope.
Not all relics remained inside the cathedral however.
Fun fact : in 1935, an archbishop placed the relics of Saint Denis, Saint Genevieve and a thorn from the crown of thorns in a weathervane at the top of the cathedral's spire to protect the congregation from lightening.
The relics were all saved from the fire and are now kept at the Louvre.