The Falcon and the Condor

Two ships that travel together have discovered a ringed gas giant planet that is the home of a cloudborne civilization. Indeed there are cities in the clouds of its very deep atmosphere. These cities float above a planetary surface none of them have ever seen, and the floating civilization is so vast that it can contain five times the possible population of earth, And these cities are sealed against the main atmosphere, holding a Nitrogen/oxygen rich atmosphere instead. Humans can breathe there.