Cirrus City was actually an incredibly large airship, built by The Technate when they seceded from the United States in the early 20's. It was capable of flight at moderate speed, but mostly floated stationary about 20,000 feet above the ground. In the late 40's, it was destroyed over Nebraska during a Civil War Two battle now known as "The Fall Of Parity".
The airship was designed as a completely self sufficient artificial urban envirnoment capable of housing in excess of 10 million inhabitants in luxury apartments. Note: At its highest population before CWII, Cirrus City housed 7.5 million inhabitants.
Cirrus City was host to many technologies that people would find almost miraculous, even today. Energy production, food production, materials production, housing, transportation, and early electronics technology were all at a peak of efficiency and in full concert with eachother in order to provide the highest standard of life possible for all inhabitants. It was generally considered by its citizens to be the closest thing to Utopia that Man had yet achieved. Sadly, most of these technologies were lost when Cirrus City crashed in Nebraska.