April 15, 2019, was a grim day for the people of Paris. Their beloved Notre-Dame, the imposing cathedral that had stood tall their whole lives, was burning to the ground — and it seemed, in that moment, like it would never recover. A few years later, of course, we’ve seen repair works instigated to restore the medieval monument to its former glory. But nobody could have predicted the secret that this project would reveal beneath this centuries-old building.