|"Dust Hill Zone" concept art.|
"More Sonic" is always a good thing, and certainly something I would've wanted as a kid, so let's take a look at some of the real "lost levels" (not the fan-made false ones). We're all familiar with Emerald Hill Zone, Chemical Plant Zone, Casino Night, Hill Top, Oil Ocean...etc... but what about Wood Zone? Dust Hill Zone? Hidden Palace Zone? Neo-Green Hill Zone? ... Genocide City Zone??
It turns out that all of them were actual lost levels in one way or another, but the truth isn't all that awesome. There are two genuine lost zones... Wood Zone and Hidden Palace, but they are incomplete. Wood Zone takes place in the treetops, has few enemies, is missing some floors, has conveyor belts that don't present any obstacle, and doesn't particularly lead you anywhere, but it is interesting nonetheless. Hidden Palace has a more polished Act 1 featuring caves and crystals (vaguely similar to Lava Reef Zone from Sonic & Knuckles), but it's also only half done--the Act 2 for it just has Sonic standing around behind a pile of rocks until he dies.
So here's Hidden Palace Zone... with it's own music and everything.
And here's Wood Zone... Wood Zone actually had a slower version of what became the Chemical Plant music, this was just placeholder music from Emerald Hill two-player mode.
That's the cool part. The truth about the other three "lost levels" is more disappointing. It turns out the equally-legendary "Dust Hill Zone" was just an early working title for what became Mystic Cave Zone (pictures of Sonic in a desert level were sent out to magazines to advertise the game, under the name "Dust Hill Zone," but no such zone was ever made). The same is true for Neo-Green Hill Zone (the original was the first level of Sonic 1). It's nothing but an early working title for Aquatic Ruin Zone. Then there's the legendary Genocide City Zone, which is just Sonic falling into a blue/green abyss and dying (and thus the name?). It's not just unfinished, it's not even started. Supposedly, the Act 3 of Metropolis Zone was going to be the one-act "Genocide City", but they designed it with the Metropolis setup instead and made it the oddball "Act 3" that is is. I always did think it was strange that Sonic would go straight from the machine-world of Metropolis to the airplane in Sky Chase Zone.
Having said that, there are a lot of cool fan-made imaginings of what the lost Sonic 2 levels "might" have looked like, including interesting and detailed Genocide City ones, but that's all they are, fan-made imaginings. They also made Dust Hill into an awesome desert level, as well as a level that looks like a precursor to the (f-ing awesome!!) Ice Cap Zone from Sonic 3. But don't be fooled by imitations. These are the authentic ones (as far as we know).