What did Thomas observe about the Maze and the Runners in Chapter 4?

Thomas makes his way toward one of the openings. Chuck stops him, telling him that the doors are about to close, just as they do every night. Thomas cannot believe that such massive constructions as these walls could be doors that move and close. Upon closer examination of the wall, Thomas can see that Chuck is telling the truth.

When Thomas looks out past the doors, he sees there are turns and corridors beyond. Suddenly, a boy turns one of the corners and is running toward the Glade. He is exhausted, drenched in sweat and looking very ragged. The boy runs past Thomas and toward the concrete bunker building near the Box elevator. Other runners are coming in from the other wall openings on each side, all of them looking just as weary as the first boy. The runners meet outside the concrete bunker door and open it before disappearing inside and closing it. Thomas is completely lost. Suddenly a loud boom startles him and the massive doors begin to close with the sound of crunching and grinding. Thomas is overcome with a sense of claustrophobia. Chuck reappears and tells Thomas to remain calm. Thomas has to fight the urge to run about past the closing doors to see what is in the Maze. Chuck pulls him away, telling it’s best to be in bed at nighttime