Vertiform City

From Memory Alpha, the free Star Trek reference

Vertiform City was a holographic city created by the USS Enterprise-D's computer in 2370. The city was a metaphorical representation of an area abundant in vertion particles, namely Tambor Beta VI, a white dwarf star. The city was so named because of the presence of vertion particles in the star. However, when the star proved to be an insufficient source of the particles, the train on the holodeck changed its destination to New Vertiform City. Presumably, the city ceased to exist once the new circuit nodes created by the Enterprise stopped functioning. (TNG: "Emergence")

Vertiform City was never actually shown on screen.