As we paused on the rim, looking toward the deep channel, Gail seized my arm and pointed. I'd seen it, too, about the same time--a long, flat barge, heaped with what looked like hay. It wasn't hay, of course, but Martian vegetation, cut and stacked on the barge. It was being taken by canal in the direction of Pnyx. And Martians, pulling ropes tied to the barge, were the motive power. There were slaves on Mars. "Martian commerce," I said. We rolled on, paralleling the canal until suddenly, ahead of us, lay another deep cut, branching out from Chalus and running almost directly west. We had no idea how long it was, for there was no sign of it on maps. And when we decided to try to cross it in our machine we saw the reason why. The canal was empty. It was half-filled with sand from countless dust storms, and there was not a blade of Martian grass in the bottom. It was an abandoned canal. And on the other side was an ancient road. It was paved with some kind of material that resembled concrete, although it was cracked and looked as if it had not been used for years. "Do you suppose the Martians had cars?" Gail asked as we stopped on the road. I shook my head. "I have some ideas about the Martians," I said. "They're probably far ahead of us in some ways--they are built for radar and they may have other senses we know nothing about--but that barge we saw was the first actual tool I've seen. If you can call a boat a tool." "They must have had machines to build the canals," she said. "Yes, but not a wheeled machine," I replied. "I don't think they have wheels on Mars. You see, the wheel was an invention man stumbled onto when he noticed that logs roll. The plants which grow on Mars, unlike our trees, have some sides that are flat. Consequently, Martians never got onto the idea of wheels." "Then what's this road doing out here?" Gail asked. "The material is pretty smooth, even though it's been etched by sandstorms," I said. "Possibly it was used to drag things over. It would be easier than dragging things over the sand." The road angled south, in the direction we were going. I hoped that all roads led to Pnyx and followed it, even though it led away from the canal. Beyond the canal we saw a round, symmetrical mound on the horizon, looking like half an overripe tomato sticking out of the sand. It was the dome of some Martian city stuck out here by a dry canal. But it wasn't Pnyx and we saw no sign of life around the town. We approached the city apprehensively. Possibly we would have avoided it altogether had we seen some sign of life, but the city looked as if there was no one at home. And the highway showed no signs of travel. In places where it was covered with sand, there were no tracks of padded Martian feet. I saw that the road led through a large arched door. "A ghost town!" Gail whispered. "An ancient, abandoned city!" "I hope it's abandoned," I said. We rode on. We had to find out, even at the risk of our lives.