It seems that only a small portion was ever implemented and did not get too far. Going by early concept art which shows roughly what the level order would have originally looked like, and lines up rather closely with the internal order, this slot was going to be used for a level called 'Ocean Wind Zone'. Most curiously, Mystic Cave is also unmentioned in those concepts, and is called Dust Hill here the latter name is mentioned in the early timeline and assigned to what seems to be a crumbling Green Hill.

Additionally, Neo Green Hill doesn't appear in any of those timezone maps. However, there are some deviations from those concepts even at this point in development: it appears that Zone 05 was likely going to be Tropical Plant, and it's Metropolis 3 both here and in the final. This earlier Zone order seems to largely match the order seen in early concepts for the game which involved time travel. Zones that are either placeholders or leftover from a previous build are in italicsnew levels are boldand moved level slots are blue. The prototype is named after Simon Wai, who found it on a Chinese Geocities page in The actual ROM image likely came from a demonstration cart that was stolen from a New York toy fair inthen dumped.

In addition, there are a few levels not seen in the final, but most of them contain nothing. There are also many, many unused graphics that can be found via hacking. The Simon Wai prototype of Sonic the Hedgehog 2 is definitely further along than the Nick Arcade buildbut there are still quite a few unfinished elements such as level layouts and things that have not been fully implemented. This is a sub-page of Proto:Sonic the Hedgehog 2 Genesis.

Sonic 1 debug mode