Montreal's Expo 67 was the most successful World's Fair in history, a vision of the future laced with monorails and space-age architecture. Its stunning centrepiece was the Biosphere, a 76m tall geodesic dome designed by Buckminster Fuller, which remained on the site after the fair left town. For the 50th anniversary of the Expo, one designer wants a vegetated version of the dome to rise next to the original.