Every now and again the news delivers a story of such magnitude that it is hard to believe it has happened.
Collapse of the twin towers of the World Trade Center in New York is such an event. It is truly unthinkable.
The scale of the human tragedy cannot be comprehended. The towers alone contained up to 50,000 workers each day and the business district of Manhattan at 9am is one of the busiest areas in the world.
It will take time for true scale to be revealed.
This is a Titanic, a Hindenberg, a Space Shuttle tragedy, but magnified by many orders.
The repercussions will be immense.
For civil and structural engineers this disaster will represent their worst nightmares and will also take time to sink in.
Here are not one but two landmark structures in the heart of the world's biggest city which within a few hours have been reduced to a pile of rubble. A tourist destination for hundreds of thousands each year and an icon for a city no longer exists.
How could this have happened? Should this have been able to happen? What are the consequences for the security of the rest of the world's buildings?
Many questions will no doubt be asked by governments and building owners and the public around the world as they come to terms with Tuesday's events.
Answers will be demanded.
Certainly the dynamics of what happened are clear to see but could structural engineers have done anything about it?
Could or should such a high profile buildings have been better protected from collapse?
Of course it is impossible to prevent a terrorist attack and the co-ordinated attacks on US infrastructure in New York, Washington and Pennsylvania bring home the awful realisation that no nation is safe from the will of any group passionate enough to commit such atrocities. History is after all littered with the results of such actions.
However, in the aftermath of this week's news we must all take a hard yet sensible look at what can realistically be done to prevent the collapse of buildings under such attack. No doubt the assertions by engineers that structures across the world, such as those at Canary Wharf, are safe from a similar catastrophe are correct. But for those working in the skyscrapers it must come as little reassurance.
Having seen the pictures from New York you could understand if many people thought long and hard about going back to work in the world's megatowers.
As an industry we must of course learn from what has happened and make whatever adjustments are necessary to design codes - if any turn out to be necessary.
More importantly, we must think about the human side to this tragedy and the work to restore public faith in the infrastructure icons that we help to create. This will mean first working to understand and explain what happened at the World Trade Center and then working even harder to ensure we convince the public and building owners that such buildings are still safe places to live and work.
Guaranteeing public safety gets harder and harder as the lengths terrorist organisations are prepared to go to increases.
As engineers we must lead the calm voice of sanity over the next days, weeks and months.