I combined two types of concept plan in this list that are very different but share the fact that they show an alternate universe where airports weren't giant fields on the edge of cities.On the one had there are the airports that literally float on water, and although these have become a reality with projects such as Kansai or military aircraft carriers, some of the original designs are for runways floating on rivers right in the middle of cities. Here the concept overlaps with the other type of floating airports: those that metaphorically float above the city on stilts - or over rather than on a river, via a structure like an overhead railroad.These concepts are not as unpractical as they appear, by using short-takeoff, quiet planes, London city airport is very close to the financial center of London and its a shame that aircraft haven't been developed to allow some of these magnificent, early ambitions to have become a reality.