Like many video games, Sonic the Hedgehog 2 is missing content that was cut before release. In the case of the blue blur's second Mega Drive / Genesis adventure, the stuff left on the cutting room floor includes entire stages – many of which never got past the conceptualisation stage and remain locked to their hand-drawn design documents.
The Video Game History Foundation's Frank Cifaldi is aiming to change that; with the help of @ManiaHez, he is painstakingly reconstructing the missing stages from the game in a process he likens to video game "archaeology."
As you can see, entire stages are being given solid form, but Cifaldi is quick to point out that just because these levels are being 'resurrected', it doesn't mean we'll get to play them one day:
So, all we have is the level layout and a general idea of the palette and art direction for the tiles. We don't know how the objects were supposed to behave, or what enemies were supposed to be here. It will never be TRULY playable with what we have.
Even so, this is incredible work which should help give fans of the game an even deeper understanding of its design and creation.