Nodes were a combination of intersections and rest areas in the outer caverns, where paths crossed, or just where there was a place to stop and catch your breath before continuing. These nodes were circular in shape, and fairly large, encased - as everything was - in a thick sheath of Nara. Each node was equipped with automatically sealing air-tight doors, so that in the event of a cave-in or other catastrophe, the doors would seal shut and only open after a certain length of time had passed without incident. In addition, a set of fans were built into the middle of the floor of every node, serving as a source of fresh air. These fans were controlled by machinery located near the base of the Great Shaft.