A neat fact I recently learned: the moment of greatest population density (at least in Manhattan, rather than NYC as a whole) was 1910, when it was 2.3 million. Today it's only 1.6 million. It's mostly due to subways and widespread car ownership which allows people to live at a much greater distance from their place of work. It's hard to imagine that many more people squishing into Manhattan than are already there today, particularly given that buildings were generally shorter!