Lighting candles for the dead in Sweden
Few will forget their first sight of a passenger jet smashing into the World Trade Center in New York, and the subsequent scenes of carnage and confusion.
Few have been left unmoved.
This uniquely awful terror attack will be indelibly etched on our collective consciousness.
Across the world shared moments of grief have happened before, as with the assassination of John F Kennedy and the death of Diana, Princess of Wales, though nothing stands comparison with this week's awful events in the USA.
Even for those not personally touched by the tragedy, it is genuinely shocking that innocent people can die while sitting at their desks, that terrorists can wilfully attack civilian targets.
As people the world over grapple with their sympathy for the victims and their anger at the as yet unknown perpetrators, condolence books and messages boards fill with words for the dead.
But there is uncertainty. Who did it? Why? Could it happen again?
Shaken and shaking
Christine Kalus, a clinical psychologist specialising in palliative care, says the fact the attack was man-made and happened in a modern city has given it a more profound impact than any natural disaster.
"The scale of loss is similar to that of an earthquake or flood but nations such as ours rarely experience natural disasters.
"This was terrorism and we find the nature of these deaths shocking. It has changed our sense of being safe in the world."
But she says this is also an event which has polarised reactions within cultures, such as the sense of triumphalism in parts of the Arab world.
Ms Kalus says: "After a relatively short period of time, we'll get back our sense of equilibrium. But for the survivors, the rescue workers, it's going to take much longer - if ever - to reach that."
Caught up in emotion
Professor David Alexander, of the Aberdeen Centre for Trauma Research, says the uncertainty over who and why the attacks took place has made many people - both in the US and abroad - feel vulnerable.
"Anger lurks close beneath the surface - people want to know who caused this.
"We want to know who the victims are and who the perpetrators are."
But rather than indulge in an outpouring of collective grief, it is important to try to understand how those genuinely shadowed by the tragedy must be feeling, he says.
Man's inhumanity to man
The Rev Jim Thompson, the Bishop of Bath and Wells, says fear is rumbling up from the ruins of the World Trade Center.
"The mayor of New York said the casualties would be more than any of us could bear.
"This volcano didn't come from below but above, raining terror on Manhattan. Not a natural disaster, but man-made.
"Will the appetite for retaliation lead to indiscriminate reprisals?"
As the US ponders its response to the attacks, and world leaders pledge their support in hunting down the perpetrators, only history will tell.