With those facts in mind, there are two possibilities: Dust Hill Zone is a level we've never seen before, it's name a reference to the fact it's a ruined present version of Green Hill Zone. The fact that that name is used for Mystic Cave Zone (and because of that, reported as it by every magazine) is purely coincidential, possibily because MCZ was written in the same slot as DHZ after it was canned. Dust Hill Zone is the prototype name for MCZ, this is supported by the name used on the prototypes, and the fact that caves are dusty. Also, they kept the name after reworking the level select screen. I am personally more inclined to the first option, even more after GoldS observation that in some contexts, ??? has been used to mean "garbage" or "junk". I can't connect visually DHZ to the final MCZ. But, we can't rule out the other option with the info we currently have. Contradicting info, even. Yasuhara claiming DHZ was the desert level, when the concept art released says it was named Sand Shower Zone, still bugs me.