Freedom Tower will be the biggest building at the World Trade Center site in New York, which was destroyed by the attacks five years ago.
It's supposed to be finished by the year 2011. The tower will rise 541m (1,776ft) into the sky and include a number of things that will make it as safe and secure as possible.
THE FREEDOM TOWER
1 The tower gets its air by sucking it in using special machines that are really high up where the air is clean
2 Central upright section for key safety features, including water-proof lifts, special fire-escape stairs and a separate staircase for emergency workers
3 Extra-strong 1m (3ft) concrete casing protecting the central section and sprinklers
4 Glass wall to protect building from explosions
A ledge at 417m (1,368ft) marks the height of the destroyed twin towers
Safety features include:
EMERGENCY CENTRAL SECTION
An upright section will run through the centre of the building, containing many of its safety systems.
It will have 1m-thick (3ft) walls made from extra-strong concrete. In it will be emergency communication cables, shafts to carry air, water pipes, and stairs and lifts to go in or out of the building.
Inside the central section there will be a special staircase for the for police, firefighters and ambulance workers to use.
Special technology will keep any smoke out, and will be extra wide to allow firefighters to carry equipment up and down, to avoid a repeat of the crowding on staircases that happened on 11 September, 2001.
The central section will also contain lifts, made water-proof to prevent damage from water sprinklers, and two escape staircases, which are joined together so if one gets blocked the other can be used.
The stairs have four street-level exits allowing a quicker escape.
When the planes hit the Twin Towers the fire-proofing wasn't strong enough to stay connected to the steel frame. An expert report said this was a big part of the collapse of the towers.
For the Freedom Tower, concrete containing strengthened fire-proofing will be used. The building designers say it could survive a very strong crash.
In 2001, after the planes had already caused heavy damage, the fires inside the Twin Towers weakened them even more.
Freedom Tower will be built so that for whatever reason, if some of its supporting structure is badly damaged, the building will stay standing, according to its designers.
Air for the offices in Freedom Tower will be taken in from the top of the building, where it is cleaner than on the street. Technology will keep the air clean even if there is a terrorist attack.
At the bottom of the building will be a wall made of three layers of a special type of glass, behind which is a very strong steel structure.
Building designers say these glass walls can survive and protect against blasts, and that the special glass will not break off into dangerous sharp pieces.
The building will be built at the north-west corner of the 16-acre (0.06sq km) World Trade Center site, between Vesey Street, West Street and Fulton Street.
It is set back from West Street, a busy road, by an average of 90ft (28m). City officials were worried that explosives inside a vehicle on the street could put the building and the people inside in danger.
Vesey Street and Fulton Street will both have limited access: only authorised vehicles can use these roads.