- "That crap must keep gelling around your face, then eat into the flesh of your neck until it cuts clean through it. Nice. That is really nice"
- — Minho
Metal balls appeared in The Scorch Trial when the Gladers were going through a completely dark tunnel after traveling through the Flat Trans. The ball is a perfectly round sphere that is programmed to detach from the ceiling and latch onto a person's head, decapitating the victim once the ball's molten metal consumes the entire head.
After walking blindly for so long, Thomas hears someone in the front screaming at the top of his lungs and squeling which reminds him animals back at the Blood House in the Glade. Soon he hears a sound of a body thrashing on the ground. Without hesitation he moves forward, pushing Gladers who were frozen in fear to the sides. When Thomas reaches out to help the boy in pain to stand on his feet, he finds out that there was no head where it is supposed to be. Not even a neck. Instead, he feels a very smooth ball of cold metal, larger in size than a head. The boy stops twitching in an instant he touches the metal ball. Thomas' mind spins to clarify if the boy's head had turned into a metal ball or it was just cut off. Winston adds the victim's voice sounded like Frankie. Later, the group of Glader run nonstop while two boys Thomas had never spoken to, get caught by the balls. At the time when Minho find stairs which lead to an open air, Winston down the stairs all of a sudden screams. A big round mass of a thick silver fluid on the ceiling grows larger and larger than in a matter of seconds it forms into a sphere of molten goop and detaches. Defying the laws of physics, it flies horizontally to Winston's head. The molten metal covers the top of his head, eating away the part above his ears.Thomas hurries to help him and manages the detach the silver goop. After the molten metal falls to the ground, it quickly forms back into a sphere then disappears into the darkness of tunnel before taking some time standing still as if taking a last look of its victim.
-Frankie dies in the tunnel.
-Two Gladers, whom Thomas has never spoken to before, die in the tunnel while running.
-Winston, but Thomas successfully manages to get the killing molten metal off his head.