- "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 Trials when the Gladers were going through a completely dark tunnel after traveling through the Flat Trans. Created by WICKED, the strange metal detaches from the ceiling and latches onto a person, flowing over their entire head and eventually decapitating the victim once the ball's molten metal consumes the entire head.
In the book, as the Gladers are walking through a pitch-black tunnel, Frankie starts screaming at the top of his lungs and thrashing around. Thomas pushes forward to try and help. Thomas reaches out in the dark and finds that there is 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 the instant he touches the metal ball.
Later, the group of Gladers run nonstop while another unnamed boy get caught by the balls. As the Gladers make their way up some stairs to the outside world, Winston, near the bottom of the stairs, begins to scream. A big round mass of a thick silver fluid on the ceiling grows larger and larger, then, 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 and disappears into the dark tunnel, but not before pausing in mid-air as if taking a last look at its victim.