The Green Hill Zone is a luscious, grass covered area of Sonic's world in the Another Time, Another Place zone.
Sonic has visited this zone numerous times during his adventures.
At one point in his younger years, Sonic was running through the zone not long after his latest victory over Doctor Eggman when the Time Eater suddenly appeared in the sky over the zone and enveloped the whole area in white. Later in the present day, Sonic found himself in the same white void after the Time Eater crashed his birthday party and kidnapped his friends. Spotting a portal to the Green Hill Zone up ahead, he headed through it to begin his journey to rescue his friends, as his younger self looked on nearby. (SSSM: #7, StH: #230)