Throughout history, there's been one form of structure synonymous with great public events - the dome. Domes have an almost mystical ability to create a sense of awe in the humble visitor, and to engineers, meeting the demand for ever larger domes had been an enduring challenge. We visit the dome of the cathedral of Santa Maria del Fiore, which marked the arrival of the Italian renaissance, and the Millennium Dome in London, the largest dome in the world. How Did They Build That? is a series that explores how new materials and construction techniques have allowed engineers, builders and architects to build some of the greatest man-made structures of all time.