Hubert Hitier / AFP / Getty

PARIS—It was Holy Week and nearing end of day, and the setting sun was as fierce red-orange as the terrible blaze engulfing Notre-Dame—Notre-Dame!—when the spire, spindly and delicate during its long life and now consumed by flames, collapsed. It was near 7:50 p.m. The sky was still light.

I was standing in a hushed, pained throng along the Quai d’Orléans of the Ile Saint Louis facing the back of the basilica, and when I watched the spire fall, I gasped and choked back tears. In this, I was not alone. Billows of pale yellow smoke rose from the nave. They became iridescent against the sky. The firefighters spraying plumes of water onto what was once the roof seemed tiny against the enormity of the structure.

How could Notre-Dame be burning? How could Notre-Dame, which had survived for eight centuries—survived plague and wars of religion, survived the French Revolution, survived the Nazis—be falling? Notre-Dame, the heart of Paris, not only a Catholic site but the preeminent symbol of European cultural consciousness, the heart of France, the kilometer zero from which all its farthest villages are measured—how could this majestic structure collapse so fast? I looked around at the faces with me in the crowd. Written on them was sadness, and pain. But also curiosity. A few giggles, as if the enormity of the loss had not yet quite settled.

The silence was interrupted by the clicking of camera shutters. And then there were the cellphones. Hundreds of people filming, photographing, sharing the tragedy, so many that the networks were jammed. Trying to capture in a few pixels what had stood for centuries, a symbol of endurance, of architectural achievement. Built in the Gothic era, destroyed in the social-media era.

The authorities will now investigate. The basilica was under construction. How maddening to see the metal scaffolding still standing, while the nave itself still burns. Nearby, people in cafés were watching the blaze over drinks. Out of sight of the basilica, they streamed images of the flames on their phones.

To those of us who live in Paris, Notre-Dame is as familiar as a landscape, and as solid as a mountain. How could it have burned so fast? I walk past it so often. I like it best at night, when the sculptures on the outside come alive under the spotlights, the gargoyles and saints and the few fallen angels plunging upside down from heaven above the central door.

President Emmanuel Macron visited after dark to survey the scene. The mayor of Paris, Anne Hidalgo, spoke on television of seeing the flames from her office window in the Hôtel de Ville, and feeling “powerless.” This loss will leave a black mark on these officials, even if the fire wasn’t in their hands. As I write this, more than 400 firefighters are fighting the blaze. It has not yet been subdued. On French television, a historian of religion, Jean-François Colosimo, described the scene as “images of the end of the world.” The fire, he said, seemed to communicate “the extreme fragility of our situation.”

Messages come in from friends around the world—“Are you okay?”—as if this were another terrorist attack, or a death in the family. In a way, it is a death. In the human family. We are all shocked together. On television, they’re showing the embers of a burning scaffolding. The sparks and ash float into the night air, like fireflies.

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