The Forbidden Zone in the Planet of the Apes movie series is the barren, lifeless area declared off-limits to all apes. While most apes do not know the precise reasons why the area is forbidden, it is generally understood to be a wasteland, one fit only for humans, outlaws, and fools.
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 for the rise of the apes.
Taylor discovers in the second movie (mostly offscreen) that there are indeed mutant humans dwelling in the Forbidden Zone – living underground, among the ruins of New York. Their spiritual center is St. Patrick's Cathedral, with the crosses on its doors turned upside down to form instead the Sign of the Bomb. At the cathedral's altar is the mutants' "Holy Weapon of Peace" – the Alpha-Omega (ΑΩ) Nuclear Device, left over from the 20th century and still functional, and capable of destroying all life on the planet, though the mutant humans appear unaware of this.
Tracing Taylor's path (even from outer space) is another astronaut named Brent, who meets Nova returning to Ape City and Zira. Brent eavesdrops on a meeting of the apes council, addressed by General Ursus, an ambitious gorilla leader. To Brent's mounting horror, Ursus begins by reminding the apes of recent crop damage caused by wild humans, who have been driven into ape territory by a lack of available food (caused by drought). Ursus has heard reports of strange sights and experiences, from ape scouts patrolling the edge of the Forbidden Zone, including one scout's report of intelligent humans capturing and questioning him, through telepathy, mind control and imposed hallucinations.
Dismissing the more sanguine aspects of the reports, Ursus presumes aloud that these humans, if they exist, must obviously have a food source, either beyond or inside the zone – and he proposes to claim it for the apes, to replace their lost resources. Ursus declares his army capable of withstanding anything the humans and their habitat have to dish out, and that the "once-forbidden zone" must be reconsidered, with the taboo against it centuries old. Dr. Zaius disagrees for the most part, but assents to join the gorilla army on an expedition.
After meeting Zira and Cornelius, Brent and Nova return to the Forbidden Zone one step ahead of the gorillas, and are led in gradually to the underground city through the wrecked Queensboro Plaza. They encounter the mutant leaders, who separate Brent and Nova and interrogate Brent telepathically. Brent and Nova attend a prayer meeting held by the mutants, who "reveal their inmost selves" to the bomb on the altar – removing masks and wigs to show themselves as grotesque, with no hair and little epidermis, and major arteries exposed; the end-product of generations living in the radioactive ruins, and a tradeoff for their superior psychic abilities.
Detecting the oncoming gorilla army, the mutants unleash their cache of psychic weapons, showing the apes visions of other apes painfully crucified upside down, surrounded by a ring of fire, then a giant statue of the Lawgiver that begins to bleed, then crackle and topple onto the ape army. Sensing a trick, Dr. Zaius rides into the middle of the visions, emerging unharmed. Their primary defense proving ineffective against the apes, the mutants each "take cover in his private shelter", while their leader (named Méndez) orders the bomb armed, aiming it toward Ape City. While Taylor and Brent conclude that the mutants don't know how powerful their weapon actually is, the apes and mutant humans prepare to battle to the death, and the whole planet may become a Forbidden Zone.
While never seen onscreen, part of the backstory to the third film is the work of renegade genius chimpanzee Dr. Milo, who readily includes the Forbidden Zone among his technological research resources. Discovering Taylor's submerged spacecraft, Milo raises and restores it, and "half-understood it", as Cornelius recalls later. When the end of the ape world appears imminent, Milo, Cornelius and Zira don space suits, board the craft, and attempt a launch – only to go through a reverse of Taylor's journey, ending up in Earth's human-led past.
Taking place in the apes' prehistory (in the latter days of human civilization), there is no Forbidden Zone in the fourth Apes movie, but chimpanzee leader Caesar predicts its coming – "when the sea is a dead sea, and the land is a wasteland!" – as he proclaims the birth of the ape society.
In the final Apes film, the Forbidden Zone appears as the Forbidden City – the radioactively contaminated wreckage of an unspecified city in North America which contained the Ape Management Complex wherein Caesar was processed, which the apes and a handful of humans escaped. Caesar declared the old city forbidden, when a nuclear war destroyed most of the Earth's human population, but later learns that videotapes of his parents (Cornelius and Zira) might still exist, and journeys back with MacDonald and Virgil to search for them. Encountering human survivors (already showing signs of mutation) under the city, a fight is sparked off between the survivors' army (using surplus weapons and scavenged civilian equipment) and the apes (with their small armory), with Ape City's human contingent corralled to keep them out of it.
When the mutant army fails to conquer Ape City, the order is sent back to launch their secret weapon against the apes – the Alpha-Omega Device. Succeeding leader Méndez, however, countermands the order, reminding the survivors that if the bomb is used, everybody loses, and tells them if they instead preserve the bomb and revere its power, they'll always have a sense of purpose. (Presumably they heed his words, and become the society encountered in Beneath.)