The World Trade Center was built on revolutionary design principles. It turned conventional architectural and structural techniques on their head. Built from a thin web of steel, its design was efficient, cost-effective and would inspire a new wave in modern building techniques. The result was two towers that were both lightweight and strong. When they were completed they were the tallest in the world. They were also milestones of architecture for another reason. The two towers were the first skyscrapers explicitly designed to withstand being hit by a jet plane. Although they had considered an aircraft impact, the designers of the World Trade towers had not anticipated the effect of an aeroplane's fuel load. British-born survivor Paul Neal tells how he smelt jet fuel rushing through the lift shafts close to his desk. "I recognised it because I'm a private pilot. I recall smelling it and instantly dismissed it as being illogical because it didn't have any place in the World Trade