"No," she said, answering me in full a half-minute later, when the door of the hospital room had been firmly closed behind us and we were committed to the big risk and there could be no turning back. She paused an instant to urge me to be cautious, to stagger a little and grip her arm for support and try to look in all respects like a patient taking his first uncertain walk after a minor operation. I didn't have to worry about looking pale, but when she went on and explained what she'd meant by the "no" relief swept over me and probably marred a little the impression it was important to give anyone who chanced to glance our way. "There's no desert to cross," she said. "It's all built up. You'll be passing between high stone walls with massive metal grills set deep in the stone most of the time, with here and there a gap and a few scattered pre-fabs occupied by aereator-system workers and their families." So that was it! I knew all about the Martian aerator-system and the big turbines that pumped oxygen out over the Colony. So much oxygen, under such stabilized pressure, that it stayed in equilibrium and didn't fly off into space even under the light gravity. Even without the aerators there was enough oxygen in the thin Martian atmosphere to enable a man to stay alive for a short period, if he didn't mind going about with his shoulders bent, gasping for breath and turning blue at intervals. His cheeks, anyway, with the veins on his forehead standing out like whipcords. The first colonists, as everyone knows, went about with oxygen tanks strapped to their backs and took a whiff or two of the stuff in Earth-atmosphere concentration through a flexible metal tube whenever their lungs started burning. And inside the early pre-fabs, of course, there were miniature aerator systems which made living indoors as comfortable as it was Earthside. But the big aerator-system had completely eliminated the need--a health hazard-diminishing need at best and never actually mandatory--of the huge glass dome which imaginative science writers in the first three decades of the Space Age had predicted as a _must_ for successful Martian colonization. There are seldom any _musts_ when science advances in seven league boots and you're right on the scene in person, breathing in a planet's atmosphere for yourself and finding out that there just happens to be a little more oxygen in it than precision instruments on Earth had led you to anticipate. It wasn't a precision instrument of any kind I was needing right at that moment--even to reassure me about my heart beat. I knew exactly how fast it was beating--much too fast. We passed a doctor in a smock so spotless it didn't seem as if he could have been wearing it for longer than a few minutes. But the look of quick suspicion he trained on us was ageless, the kind of look that comes into the eyes of a trained professional man when he can't be quite sure that a subordinate is doing the wise thing.