The world's leading experts boldly attempt to build, for the first time ever, two of Leonardo da Vinci's machines to the exact specifications that he designed 500 years ago. The first is a giant crossbow, with arms nearly 70 feet across, and the second is a glider that predated flight by 400 years. This two-part special reveals whether the inventor's ideas were flights of fancy or revolutionary designs hundreds of years ahead of their time.