Yes, he knew all right! Nature had been probed in its darkest lair by a clumsy hand. Nature had growled back threateningly. It had almost bitten. Almost...? Sam Conway's ribs seemed to shrink about his wildly pounding heart. He leaned against the cracked brick wall, trembling. In memory he was on Mars again seeing those ruined buildings, sheered off, buried by the dust--smelling the metallic reek of ozone that had seeped back through the breath-vent of his oxygen helmet. Even as here, now. Ozone built up from the commoner form of oxygen by electrical discharges! And by swift suggestion, Sam's thoughts went beyond Mars itself. Outside of the Martian orbit was the Path of Minor Planets--the asteroids. Broken up fragments. Perhaps a single world, once, that had been caught in catastrophe.... There was more, too. What were the rings of Saturn? What cataclysmic circumstance had made them? Atlantis and Mu, the lost continents. Why had they sunk beneath the sea, taking with them their splendid civilizations? And there were the novae far out in interstellar space; normal stars suddenly blazing forth in spectacular ruin. Yes there must be many other inhabited worlds in the universe, other folk, studying, learning to control and curb matter and energy. Sometimes knowledge must get dangerously ahead of itself, lacking a sound foundation of understanding. And then? There was silence outside the building. So the crunch of hurrying footsteps in the cinders of the driveway penetrated easily to Sam's eardrums and excited nerves. A loud knock sounded at the outside door of Sam's sleeping room. He staggered back from his ruined laboratory. From a small chemical cabinet he procured a flashlight. And he drew the pistol he always carried now, from his pocket, before he unfastened the heavy bar of the door. It was Ellen Varney out there in the dark. Sam hadn't seen her in almost a week. He had never permitted her to come here when he was busy. To the rear, down the driveway, the headlamps of the girl's car made a white lantern-glimmer through the bushes. For one frightening instant Ellen saw the pistol muzzle levelled toward her before Sam was able to recognize her and lower the weapon. But she didn't ask the reason for the gun at all. "Sam," she stammered. "I couldn't sleep and I heard a funny, sharp explosion. It seemed to be in this direction. And when I looked out of the window I saw a glow in the sky--very faint. But it was in this direction too. I guess I had a hunch, so I drove out here. All the way I could smell ozone in the air. You can hardly see the phosphorescence in the sky from up close at all. But it's right over. What's wrong, Sam? What have you _really_ been doing?" The girl's tense fears, strong enough to make her come here, after midnight, to his laboratory, emphasized Sam's own private anxieties.