The Eiffel Tower. The Seattle Space Needle. Seville’s Puente del Alamillo. These and other structures built for World’s Fairs are among the most iconic on the planet. The Universal Exposition has long been a place for architects to display their boldest ideas, play with cutting-edge materials and pull off incredible feats of engineering and design. At this Expo 2015, which opens May 1 in Milan, the architects chosen to create pavilions were asked to focus not only on innovation, particularly around the use of technology, but also on sustainability.