In pictures: Replacing the twin towers

1 of 8 The World Trade Center's original twin towers stood for almost 30 years and cost $400m to build.

2 of 8 Architect Daniel Libeskind won the competition to design their replacement.

3 of 8 But concerns about a lack office space forced a redesign.

4 of 8 Architect David Childs made the modifications to Libeskind's design.

5 of 8 Despite disagreements over the designs, there was a show of unity when the final model was unveiled.

6 of 8 The new building will dominate the New York skyline by day...

7 of 8 ...and by night.

8 of 8 But at the site of the destroyed towers, a new building seems a long way off.