Charlie Luxton travels to Commerzbank, Frankfurt to explore the world's first eco-friendly skyscraper, designed by British architect Norman Foster.

Still the tallest tower in the US – as well as the tallest building in the world for 25 years – the $175million Sears Tower broke boundaries in not only height but design and construction when it was completed in 1973. But building the tallest building in the world brings its own logistical headaches – and the crippling cost of funding an iconic tower can take its toll. Presented by Keith Keaveney. Architects: S.O.M (Bruce Graham)