The thin white shape of another aërocar rushed past us overhead. We were descending now. I had guessed the surface to be some ten thousand feet beneath us. We dropped lower. I could make out a rocky, undulating landscape. Occasional patches of what might have been soil. Shining, narrow ribbons of roads. Areas of vegetation. We passed over a village. Dull spots of light, merged into a glow. I saw the dark shapes of houses; on a hillside, tiers of them. There was movement down there, in city streets. Off to one side, beyond the settlement, a great flat structure was bathed in a red blast of light. It seemed a factory. A pit in the rocks beside it glowed red. We swept on. The settlement vanished behind us. I saw a point of light, like a beacon, set on the summit of a rocky cliff. It changed color at intervals. Entt remarked it, with a gesture to Tad. He swung the controls; we went into a sharp, upward climb. There were points of light always showing in the black void over our heads. As we had descended toward the rocky landscape, the lights overhead had grown very dim. I gazed up at them. They twinkled up there, very faint and dim now. I wondered what they could be. Not aërial beacons, poised over us? As we climbed, they began to brighten. My imagination struggled to cope with this I was seeing. This silent realm down here--I had the sense of a great celestial spaciousness, but I knew that it was not so. This was within our earth, underground; a great, black void here, like a titanic cave. Yet it must be of finite area; comparatively small. Over my head now--up there where the points of light blazed like stars--must be some great rocky ceiling. And above that, miles above it, no doubt, my imagination saw the floor of our Pacific Ocean! We ascended in a steep slant. The upper stars brightened. The lights beneath dimmed with distance. Then I saw overhead the outlines of what indeed was a rocky ceiling. It spread horizontally over us; eight or ten thousand feet still up there, at the least. I saw the lights set in this rocky ceiling. * * * * * And then I gasped. With sudden, changing viewpoint, I saw what was the truth. There were ribbons of roads on the rocky ceiling. Patches of open space that might have been soil. An open area glowing with light; houses in it--a settlement! It hung up there, the distant, small image of it--a settlement of houses and streets, upside down, perilously clinging to our ceiling!