The Forbidden Zone was declared by The Lawgiver early in the apes' history. While the land had once been fertile, "the beast Man" had destroyed it through carelessness; anyone remaining there long was condemned to an early death.
When the spacecraft (called the Icarus) carrying Colonel Taylor and his team of astronauts crash-lands in a lake, they cross the Forbidden Zone, noting both that "nothing will grow here!", and "that strange luminosity at night, and yet there's no moon" on their way. Later they find a row of markers like scarecrows lining the edge of the zone. Not far on the other side, life blossoms – and the apes reign supreme.
Taylor meets the chimpanzee Cornelius later, and describes his land journey to Cornelius thoroughly enough that Cornelius knows the specific area where Taylor landed, and it isn't far from where Cornelius had conducted an archaeological dig the year before. (The dig had been stopped by the Ministry of Science, headed by orangutan Dr. Zaius, to whom Cornelius was subject.) Dr. Zaius also learns that Taylor came from the Forbidden Zone – but believes him to be a native of it, and probably a member of a mutant human tribe. He tries to persuade Taylor to disclose information about this tribe, but Taylor can't tell Zaius anything he wants to know, so Zaius plans to have Taylor first gelded, then sent to an ape laboratory for vivisection or other experimentation.
To save Taylor, Cornelius and Zira risk their careers (and public lives) by first breaking Taylor out of his holding cell, then taking him back to the Forbidden Zone, to the site of the dig, intending to let him go and come back with proof that the apes don't know the whole truth about humans, nor their past. Dr. Zaius follows them with a squad of gorilla soldiers, but is captured by Taylor. In the end, Zaius bids Taylor go free in the Forbidden Zone, telling him "The Forbidden Zone was once a paradise. Your breed made a desert of it, ages ago!"
When Taylor wonders about the root of the ape/human question, Zaius says confidently, "Don't look for it, Taylor – you may not like what you find." Taylor and Nova ride off together into the Forbidden Zone, and ultimately come upon the wrecked remains of the Statue of Liberty. Taylor realizes what Zaius knew (at least in part) and kept hidden all along; the Forbidden Zone is the irradiated dead zone around what was once New York City, destroyed in a nuclear explosion as humanity destroyed itself, allowing the rise of the apes.