Once again, we’ve got someone using a possessive instead of a plural. *sigh* The most elite community’s what, then?
Underground and under ground do not mean the same thing. The first is an adjective, and the second is an adverbial phrase: Underground cities are located under ground.
Notice that a comma has been added after in the sky. This is not because there’s a list of three items (locations) before it, but rather because each of those three items is an introductory phrase, and you’d need a comma after even one of ’em. (Even if the sentence began with just Under ground, these cities have been…, you’d still need a comma after the introductory phrase.) The commas separating them are because they’re a list, though.
Another comma has been added before until now.
Also notice that I changed the fake “ellipsis” at the end of the example to a period. If you want a sentence to trail off (although it wouldn’t make sense for that one to do so), use a real ellipsis with three dots.
Some of the most elite communities of the world have, over time, constructed massive, secret cities for their personal use and recreation. Under ground, under water, or in the sky, these cities have been hidden from mankind for centuries, until now.