Sixteen years after the world-changing attack on the World Trade Center in New York, sometimes it still does not seem real.
We've seen images of the planes exploding into the soaring skyscrapers and the towers crashing down so many times, yet the events of September 11, 2001, remain hard to fathom.
A visit to the impressive 9/11 Memorial and Museum in downtown Manhattan changes that. The museum, which has welcomed more than 10 million visitors since opening in 2014, commemorates the attack in human terms.
Encased in glass are hundreds of frankly mundane items - receipts, computers disks, shoes, a backpack, a torch - that were salvaged from the wreckage. When viewed together, these items remind you of the everyday people who perished, people who had just arrived to the office to start their day when the unimaginable happened.
Among the mangled pieces of metal and twisted-up emergency vehicles inside the museum is an exhibit so shocking that it was placed behind a wall, giving visitors the choice as to whether they want to see it or not.
Behind the wall is a gallery of photographs playing on a loop, news.com.au reported.
They remain the most shocking images of that day's horrors. They are so confronting that newspapers, such as The New York Times, received a barrage of complaints for publishing them. Even today, the images are rarely seen in the US.
According to New York magazine, the photographs "have been rendered taboo, vilified as an insult to the dead and an unbearably brutal shock to the living".
The photos are of the people forced to make the unthinkable choice to jump out of the windows of the twin towers after the planes ploughed into them. Faced with the prospect of burning in fire on the upper floors, they instead leapt off the building to certain death.
The hard-to-view photographs in the museum are accompanied by eyewitness statements that help to make the desperate actions of these poor souls intelligible.
Lower Manhattan resident James Gilroy retells how he saw a memorable human moment before unspeakable tragedy.
"She had a business suit on, her hair was all askew," he said.
"This woman stood there for what seemed like minutes, then she held down her skirt and then stepped off the ledge.
"I thought, how human, how modest to hold down her skirt before she jumped.
"I couldn't look after that."
Eyewitness Victor Colantonio remembers seeing a man jump.
"White shirt, black pants, end-over-end tumbling to the ground," he said.
"At that instant, the towering glass and metal mass of billowing smoke became human."
Another Lower Manhattan resident, Louisa Griffith-Jones, said she "felt compelled to watch out of respect to them".
"They were ending their life without a chance and to turn away from them would have been wrong," she said.
In the decade and a half since 2001, little is known about the estimated 200 people who jumped or fell to their deaths. Most who jumped were from the north tower, most likely because the fire was more intense in that building, concentrated on fewer floors.
Eyewitnesses said some people tried to use curtains or tablecloths as parachutes; others saw a couple holding hands as they plummeted.
Perhaps most tragic is it is still unclear exactly who died by jumping out of the towers because authorities were unable to recover or identify the victims' remains.
One of the most enduring images of September 11 is "The Falling Man", shot by Associated Press photographer Richard Drew.
The image is so unforgettable that it has been the subject of countless articles and even a documentary.
Although it is not certain, the subject of the picture is believed to be Jonathan Briley, 43, who worked in a restaurant on the 106th floor of the north tower.
The New York Times published the image on page 7 on September 12, 2001, but refrained from using it or images like it ever again due to an angry backlash from readers, who found it deeply disturbing.
Today, it is still impossible to be sure how many people chose to jump. Some may have fallen accidentally, or even have been pushed out in the panic.
Regardless, none of the deaths on September 11 - apart from the terrorists - have been officially ruled as suicides.
"This should not be really thought of as a choice," then New York City chief fire marshal Louis Garcia said in 2004. "If you put people at a window and introduce that kind of heat, there's a good chance most people would feel compelled to jump."
As USA Today put it, they weren't choosing whether to die. They were choosing how to die.