The man released his foothold and slid smoothly below the floor level just as moving shadows appeared at the bend of the corridor. He dropped down the column through four more unlighted levels, reaching an atmosphere that held a blend of machine oil along with its other odors. Light filtered upward with the air currents. Somewhere below was a very bright level, whence came the rhythmic throb of heavy machinery. This did not resemble the sounds of a spaceship, nor yet a Terran factory, but some considerable work was being carried on. He groped out in the darkness for a foothold, got the other foot over, and wearily pushed himself away from the column. He was on a level so dim that he touched the edge of the floor opening with his toe to make sure of its location before moving off along the corridor. In the darkness, he went more slowly than before, but made better time than looked possible. Under the circumstances, he reassured himself by stretching out his left hand every few seconds to touch the smooth wall. He walked normally, though not noisily, and his sense of direction was extraordinarily good. About a hundred yards along a corridor that seemed not to have a single bend or corner, he slowed his pace doubtfully. A few steps more brought him to another closed door. This one, however, yielded to his shove, swinging back to reveal a stretch of tunnel with a bare minimum of illumination oozing from widely spaced ceiling fixtures. Here, he could sense side doorways his fingers had usually missed along the darker stretch. He had gone another hundred yards and finally passed two cross corridors, before he was again obliged to stop and rest. He slumped against the side wall, favoring his right arm and gazing dully before him. A few steps further along was one of the typical elliptical doorways. Through this one, some light was reflected to the wall of the corridor. The man stared at it in the way anyone in the dark will turn his eye to light. After several minutes, he moved toward it as if impelled by idle curiosity. Reaching the opening, he hesitated. A strange expression flickered over his face. The decision to look or not to look was causing him great uneasiness. Finally, he stepped forward and entered a small chamber. This was evidently located so as to house another slim column that disappeared upward and downward into unknown levels. Several small, oval windows were set just below the ceiling, at a height which presented no particular difficulty to the man when he stepped over to look through them. The scene that met his eye was a wide corridor, so wide that it might be termed a concourse or even a public square. Members of the public that were to be observed frequenting it were very, very far from being human.