The player is introduced to Sonic's world (during the early games at least) in the
Green Hill Zone(or Emerald Hill Zone), is a place near untouched by the industrial dreams of Dr. Eggman. The grass is green, and the water sparkles. As he moves further into the game, and the scenery becomes more artificial, and places such as chemical plants and casinos become the new scenery. This is much like our own world.
16 Greatest Levels in Sonic the Hedgehog History.