The Mirage Spots are randomly selected areas, with yes, rare foreign Pokemon. The areas, random every day, are always in the same location. (Weird, right?). So this is my theory, it could be some form of a reality optical illusion! The word 'Mirage' means exactly that, an optical illusion. I'm guessing this has something to do with space-time, as if you don't know already, foreign legendaries can be captured after putting your hand deep inside strange wormhole-like rings. Dialga and Palkia are caught similarly, but with a tear in the space-time continuum. The Mirage Areas are reached using the same feature as the wormhole ring locations, riding Mega Latios or Latias. Strangest of all, one of the randomized Mirage Spots is a place called Crescent Isle. It's one of the random spots. Where have we heard Crescent before? Cresselia. Yes, at this randomized island, you can catch Cresselia. Here's the last and most freaky part- the unlock method is wait for it... Wait for it... Unknown. Yes, the unlock method for the Mirage Locations are completely unknown to human knowledge.
Hope I helped! (Even if I got a bit carried away...)
Source: Experience, a Google search, and a bit of Serebii. ;)