As the world watched the World Trade Center’s collapse on September 11, 2001, it’s hard to imagine how anybody could have survived.

On a day of immeasurable destruction and loss, miracles were rare but a number of survival stories from the 9/11 terror attacks offered glimmers of hope.

One such story was that of Josephine Harris and firefighters who stayed to save her, who survived the collapse.

As unimaginable amounts of concrete and steel fell around them, stairwell B survived, and so did they.

This is their story, as told by Dennis Cauchon and Martha T. Moore of USA Today.

The miracle story of Josephine Harris and Stairwell B

Twelve firefighters. One cop. One civilian.

The firefighters and the cop had been on the 23rd through 35th floors of the north tower when the south tower collapsed at 9:59 a.m. Ordered to evacuate, they joined a line of uniformed colleagues descending Stairway B, the center stairs in the building.

The evacuation procession extended from high in the building, down the staircase, through the lobby and out to the street.

"Davey, we gotta go! Now!" yelled Capt. Kathy Mazza, 46, the highest-ranking woman on the Port Authority police force. She was yelling up the stairs.

"I'm right behind you, boss," responded canine officer David Lim, 45. Then he stopped for a moment to help firefighters who were carrying a 59-year-old Port Authority secretary, Josephine Harris.

The firefighters of Ladder Company 6 had found Harris on the 22nd floor, tired and crying, unable to continue. "Cap, what do you want to do with her?" a firefighter asked.

"We'll take her with us," said Capt. John Jonas, head of Ladder 6.

Firefighter Bill Butler, a bull of man, began carrying Harris down the stairs. Others searched for a chair that could be used to carry her down.

The rumble began at 10:28 a.m.

The noise was so loud, "it sounded like you were standing between two Amtrak trains going in opposite directions," Butler recalls.

Firefighter Sal D'Agostino jumped for the protection of doorways. Jonas hustled back into the stairway from a floor where he had been searching for a chair to carry Harris.

A hurricane-like wind blew down the stairway. Firefighter Matt Komorowski flew, literally, from the fourth to the second floor. Battalion Chief Richard Picciotto, 51, was thrown from the sixth to the second floor.

Then the noise stopped. The stairwell was dark, smoky, dusty. The men's eyes, ears and mouths were clogged with dust.

The firefighters sounded off. There were a dozen, plus Lim and Harris. The 14 who survived were scattered inside the stairway from the lobby to just below the sixth floor. Miraculously, none had life-threatening injuries.

Two firefighters who had been above and below them in the same stairwell cried for help.

Battalion Chief Richard Prunty, 57, radioed from the lobby that he was pinned under a steel beam and losing consciousness. Michael Warchola, 51, a lieutenant on Ladder 5, radioed that he was trapped on the 12th floor of Stairway B. He did not know that the 12th floor did not exist anymore. He had been thrown somewhere else.

The uninjured firefighters tried to reach Prunty and Warchola but were blocked by debris. Prunty and Warchola died. The body of Mazza, the police captain, was later found outside the building.

But the 14 people inside Stairway B from the lobby to the sixth floor were spared. Why? Nobody can say for sure, but the survivors were in a structurally unique location in the 110-story tower.

The stairwell was at the center of the building's core, a rectangular area of elevator shafts, plumbing and stairwells. On the bottom six floors, the core was surrounded by open space — a giant atrium that gave the lobby a grand look. Just above the survivors, a thick reinforced cement floor supported a mechanical equipment room.

When the towers fell, the reinforced seventh floor — like a protective helmet — helped slow the collapse just enough to divert the debris into the open air of the six-story atrium.

"When the debris hits the atrium, it has zero resistance," says Gene Corley, chairman of a federally funded study of the collapse of the buildings. "It's a viable theory that the debris diverted around them just enough to protect the stairway."

Jonas describes it more colorfully: "The tower came down like a peeling banana, and it peeled around us."

The stairway itself barely survived. Railings were bent. Debris covered the steps. But, amazingly, it was passable from the second to fifth floors.

After the dust settled, the trapped firefighters went exploring.

After three hours, they made a discovery: sunlight. Between occasional breaks in the smoke, they could see the sky from a hole on the side of the fourth-floor stairs.

Picciotto was the first out. He walked up the stairs and onto the top of Ground Zero. He was alone in an endless field of debris. Buildings burned in the distance.

Lim walked up the stairs and joined him. They stood in silence atop 16 acres of rubble.

"Chief," Lim said finally, "what do you think the chances of surviving something like this are?"

"One in a billion," Picciotto said. "One in a billion."