Building the Impossible: The History of the World Trade Center

The World Trade Center has been built twice, using two very different kinds of engineering. This documentary looks at how the original Twin Towers were designed to stand up to the wind, and why One World Trade Center was built in a completely different way on the same ground in Lower Manhattan. In the 1960s, engineers Leslie Robertson and John Skilling faced a site of waterlogged landfill beside the Hudson River and a building program larger than anything New York had seen. This video explains how their solutions worked: a 70-foot slurry wall known as "the bathtub", a framed-tube design that let the towers sway in the wind, and sky lobbies that changed how elevators move people through supertall buildings. It's a story about engineering, memory and rebuilding, told through the structures that have stood on one of the most significant sites in America. The Steel Ledger explores the engineering history behind the great cities of the modern world, from New York and Chicago to London, Paris and beyond.