Around the great compartment we walked, our eyes ranging over the great pear-shaped mechanisms and the great pipes connecting them, and then Marlin and I stopped short. For there in the black wall before us was a door, an opening that connected with the adjoining compartment on that side. It was by means of doors like that, it was plain, that the necessity of streets in this compartment-city had been obviated, making it possible to pass across the city through the compartments themselves if needed. Yet it was in stupefying surprise that Marlin and I now gazed toward that door. For it was all of six feet in width, but hardly more than four feet in height! Its opening stretched there in the black wall as though an ordinary door-opening of one of Earth's buildings had been set in that wall sidewise! And as we looked stunnedly about we saw that all the doors set in the compartment's four walls were of the same size and shape! "Those doors!" I cried to Marlin, leaning beside me. "Those were never made for human beings or for near-human beings!" "Then these Neptunians that once were here--" Marlin began, and then stopped; we gazed in silence at each other through the vision-windows of our space-walkers. And silence seemed oppressive all about us then, a silence that lay as thickly over the deserted compartment-city as the thick dust of unguessable years that covered it. A chill seemed to have struck home to us in some strange fashion with the discovery of those grotesque door-openings and their significance. I glanced upward, saw that the faceted ball of the space-flier was still hanging motionless high above, and then as I turned back saw that Marlin had moved toward the low door toward which we had been gazing. A moment he contemplated it, then motioned to me with the big metal arm of his space-walker, and as I came to his side, grasped that door's edge with his great arms and lowered his space-walker's big cylinder until it lay on its side on the smooth black paving. Then he was drawing it through the door, aiding himself with a force-ray shot from the opening in its bottom toward the compartment's opposite wall behind us. When he was through, he drew himself erect, and in a moment I had followed him and was standing with him in the next compartment.