Gandharvanagara is a concept found in Hindu books. It is an imaginary city or mirage town. Hindi Sabdasagara describes Gandharvanagara as follows:
Owing to different optical illusions, villages and cities may appear to exist in the sky and also on seas, rivers, etc. When during summer the air on the upper strata of deserts, seas etc. gets heated, expanded, and rises up, the light passing through it becomes multi-coloured and gives reflections in the sky in the form of village parts or cities. Also can be seen therein reflections of trees, boats etc. This is a mere optical illusion. This reflection immediately fades out also. This unreal city, which is an optical illusion, is Gandharvanagara.
Sage Vyasa in Mahabharata equates Sages disappearing from sight like the Gandharvanagara fading out.
Gandharvanagara, an interesting atmospheric phenomenon, is also mentioned in works on Hindu philosophy — especially the Advaita Vedanta — as an illustration of something which appears but does not actually exists.
Very rarely this Mirage City is considered as the city of celestial beings especially Gandharvas.