"That Notre-Dame still stands is due solely to the enormous risks taken by firefighters in those third and fourth hours."

"Disadvantaged by their late start, firefighters would rush up the 300 steps to the burning attic and then be forced to retreat. Finally, a small group of firefighters was sent directly into the flames, as a last, desperate effort to save the cathedral.... 'We were at first reluctant to go because we weren’t sure we’d have an escape route'... A group of firefighters from a neighboring suburb refused to go, but another team said it would do it. They broke a gate, and as they went inside the northern tower, found parts of a wall and the floor on fire. They climbed a set of stairs to the height of the bells. From there, they could douse the flames. One firefighter almost fell through the cracking steps — but by 9:45, they had the flames under control.... 'First off, this is all about our fragility,' Monsignor Chauvet, the rector, said on reflection. 'We are as nothing. The fragility of man, in respect to God. We are nothing but — creatures.'"

From "Notre-Dame came far closer to collapsing than people knew. This is how it was saved" — a detailed and elaborately illustrated NYT piece.