Newt: "Hear that? It's the maze, changing. It changes every night.
Thomas: How is that even possible?"
Newt: "You can ask the people who put us in here, if you ever meet the bastards. Listen the truth is... The Runners are the only ones who really know what's out there. They are the strongest and the fastest of us all. And it's a good thing, too... Because if they don't make it back before those doors close... Then they are stuck out there for the night. And no one has ever survived a night in the maze."
Thomas: "What happens to them?"
Newt: "Well, we call 'em Grievers. Of course, no one's ever seen one and lived to tell about it. But they're out there."