A loud boom exploded the air, making Thomas jump. It was followed by a horrible crunching, grinding sound. He stumbled backward, fell to the ground. He wouldn’t have believed it if he hadn’t seen it for himself. The enormous stone wall to the right of them seemed to defy every known law of physics as it slid along the ground, throwing sparks and dust as it moved, rock against rock. The crunching sound rattled his bones. He looked around at the other openings. On all four sides of the Glade, the right walls were moving toward the left, closing the gap of the Doors.
Then one final boom rumbled across the Glade as all four Doors sealed shut for the night.x