When it comes to the enduring images of 9/11, the horror-struck faces of those watching the tragedy unfold in front of them comes a close second to footage showing the attacks on the World Trade Center's twin towers.
That reaction - the slack jaw, the unblinking and uncomprehending stare and the hand rising subconsciously towards the face - really translated what happened onto a human scale.
That same human reaction is still happening dozens of times a day in lower Manhattan - except this time it occurs at two separate exhibitions that endlessly loop footage of the aftermath and impact of the attack.
One of them, Tribute WTC Visitor Center, is a non-profit museum set up by families of those who were in the twin towers on the day of the attacks. The other, the National September 11 Memorial & Museum, is an exhibition previewing the memorial that will open on the site of the twin towers tomorrow to mark the 10th anniversary.
Neither one features footage of either of the passenger jets hitting the twin towers but both are compelling and deeply affecting, undiminished even after all these years and all the attempts to rationalise what occurred.
The display at Tribute WTC, located directly across the road from the World Trade Center site, is sufficiently moving on its own, but they also organise walking tours, conducted by those who escaped or family members of those who did not.
On the day I was there, the guides included the mother of a 23-year-old man who had died in the twin towers and many whose work colleagues had died.
With the building site being off limits, there isn't much to see and so the commentary reflects upon what used to be there and what is in the process of being built. But that doesn't stop the tours from becoming emotional experiences, both for those leading the tour and those taking it.
There are often tears on both sides - a common reaction, based on reports of the tours. But that's the idea: to take an endlessly replayed and analysed world event and bring it down to the human scale of loss.
Inside the Tribute WTC museum, the display is made up of a series of themes - about the vibrant community centred around the twin towers before the attack; the events on 9/11; the stories of the rescue workers who rushed to the site; photographs and memorabilia commemorating the lives of those who died; and, finally, stories of how people turned their grief into action to improve international understanding rather than focusing on revenge. Scattered discreetly around are boxes of tissues, obviously well used.
As someone who once lived in Manhattan, had a flatmate working at the World Trade Center and who had not returned since the twin towers came down, I wasn't sure what to expect of the exhibition. I had steeled myself for a flag-waving and uber-nationalist place screaming "USA!" But it wasn't.
Instead, it was the same theme again and again: a human story amid a colossal tragedy. Remember those tragic fliers featuring photos and details of the missing, posted in their thousands, around the site of the World Trade Center? Replicas of them (with phone numbers obscured) cover the entire wall in the exhibition.
It starts with just one picture - featuring a description and photo of Cantor Fitzgerald worker Andrew Stern, snapped smiling in a tuxedo. The wall is painted a blemishless blue, the colour of the sky on that early weekday morning of September 11, 2001. As you walk along the wall, dozens of more fliers appear until the wall is absolutely obliterated.
Tribute WTC, like the National September 11 Memorial & Museum, deals not just with the twin towers but also feature the original World Trade Center attack in 1993, the attack on the Pentagon and of Flight 93, the United Airways jet that crashed in Pennsylvania during an attempt by passengers to regain control of the aircraft. But the proximity to the World Trade Center site means that the collapse of the twin towers dominates.
It's the same at the National September 11 Memorial & Museum, a few minutes walk from Ground Zero. Besides showing the design of the memorial - two massive recessed pools on the footprints of each of the twin towers and set amid a glade of oaks - it gives a hint of some of the exhibitions that will be at the 9/11 Memorial Museum.
This includes 9/11: Stories of Survival and Loss, an endlessly looping eight-minute film depicting footage taken on September 11 and the days that followed. More than the film in the Tribute WTC exhibition, this is the one that prompts the involuntary slack-jawed shock in the audience .
Along with the footage are the stories of those who were involved, both in the World Trade Center and also those who lost loved ones on the hijacked planes and in the attack on the Pentagon. Keating Crown, who worked on the 100th floor of the South Tower, said that, after the second plane hit, he crawled around for three or four minutes, unable to see or hear anything. "I'm thinking that this is probably it," he says in the film. "I'm really starting to get a sense of why people had been jumping out of the windows."
Tom Canavan, a securities specialist working in the North Tower, said he was descending the tower when it collapsed. "I felt a rush of wind coming down from above me and what sounded like a thousand freight trains coming," he said. "Before I had a thought to even say anything or do anything, I heard 'thump, thump, thump'. Faster, faster, and then just smack down."
New York firefighter Mickey Kross described how he was in the North Towers when it collapsed. His small part of the stairway remained intact, saving him and 13 others when those above and below him all died. "I didn't even think of being frightened," he explains. "When I came out of that hole and I saw the devastation that was around me ... Everything's gone. The World Trade Center had disappeared."
Elsewhere in the exhibition, a human-sized Statue of Liberty stands absolutely bedecked in flags, keepsakes and notes of remembrance. One message, written by the family of Cantor Fitzgerald worker Steve Russin, says: "Daddy, we miss you! Alec, Ariella and Olivia. Steven H Russin. 6/9/69 - 9/11/01".
There are other sites connected with September 11 in the area, although life in lower Manhattan mostly goes on at its usual frenetic pace. St Paul's Cathedral, the oldest building in Manhattan to be in continuous use, played its own part. Already boasting an impressive history, having been built in 1776 and hosted George Washington on his inauguration, its steel fences were one of the main sites for fliers of the missing people.
Later, London's Lord Mayor and the Archbishop of Canterbury presented the Bell of Hope, which was installed on the cathedral grounds on the first anniversary of the attacks. It has rung every year since, and also rang out in 2004 after the Madrid train attacks and the following year when London's trains and buses were bombed. Nearby, a slab of granite installed after the events of 2001 bears the simple inscription: "They are in peace".