MoMA has launched a new exhibition site called Tall Buildings. It's not quite what you'd expect — the Sears Tower and Hancock have no mention, and the Petronas Twin Towers in Malaysia is absent. Rather, the site explores buildings from across the world but more importantly, buildings that have yet to been built or were proposed but not built. From a set of criteria, weighing in at number one out of 25 buildings in the exhibition, is the Chicago building that wasn't, at 7 S. Dearborn (more info here) which would have been the world's tallest building.