In the early days, when the conquest of Mars was just beginning, Earth scientists had been wild to get into the city. They had observed it from every angle, taken photographs of its architecture and the robots that still patrolled its fantastically winding streets, and then they had tried everything they knew to pierce the wall. Later, however, when every unsuccessful attempt had precipitated a bloody uprising of the present-day Martians--resulting in a rapid dwindling of the number of Martians--the Mars Protectorate had stepped in and forbidden any further experiments; forbidden, in fact, any Earthman to go near the place. Thus matter had stood for over a hundred years, until Harold Tate. Tate, a physicist, had stumbled on a field that seemed to be identical in properties to the Kal-Jmar dome; and what is more, he had found a force that would break it down. And so he had made his first trip to Mars, and within twenty-four hours, by the blindest of chances, blurted out his secret to Syme Rector, the scourge of the spaceways, the man with a thousand credits on his sleek, tigerish head. Syme's smile was not tigerish now; it was carefully, studiedly mild. For Tate was no longer drunk, and it was important that it should not occur to him that he had been indiscreet. "This is native territory we're coming to, Harold," he said. "Better strap on your gun." "Why. Are they really dangerous?" "They're unpredictable," Syme told him. "They're built differently, and they think differently. They breathe like us, down in their caverns where there's air, but they also eat sand, and get their oxygen that way." "Yes, I've heard about that," Tate said. "Iron oxide--very interesting metabolism." He got his energy pistol out of the compartment and strapped it on absently. Syme turned the little sand car up a gentle rise towards the tortuous hill country in the distance. "Not only that," he continued. "They eat the damndest stuff. Lichens and fungi and tumble-grass off the deserts--all full of deadly poisons, from arsenic up the line to xopite. They seem intelligent enough--in their own way--but they never come near our cities and they either can't or won't learn Terrestrial. When the first colonists came here, they had to learn _their_ crazy language. Every word of it can mean any one of a dozen different things, depending on the inflection you give it. I can speak it some, but not much. Nobody can. We don't think the same." "So you think they might attack us?" Tate asked again, nervously. "They _might_ do anything," Syme said curtly. "Don't worry about it." The hills were much closer than they had seemed, because of Mars' deceptively low horizon. In half an hour they were in the midst of a wilderness of fantastically eroded dunes and channels, laboring on sliding treads up the sides of steep hills only to slither down again on the other side. * * * * *