Tom held his breath, waiting. This was a storage hold, but he didn't dare to move, even to take cover. The guard stood motionless for a moment, then grunted, and resumed his slow pacing. When he had moved away Tom caught his breath in huge gasps, his heart beating in his throat. It was no use, he thought in despair. Once or twice he might get away with it, but sooner or later a guard would be alert enough to investigate an obscure noise, a flicker of movement in the corner of his eye.... There had to be another way. His eye probed the storage hold, hopelessly, and then stopped on a metal grill in the wall. For a moment, he didn't recognize what it was. Then there was a _whoosh-whoosh-whoosh_ as a fan went on, and he felt cool air against his cheek. He held out his hand to the grill, found the air coming from there. A ventilation shaft. Every space craft had to have reconditioning units for the air inside the ship; the men inside needed a constant supply of fresh oxygen, but even more, without pumps to move the air in each compartment they would soon suffocate from the accumulation of carbon dioxide in the air they breathed out, or bake from the heat their bodies radiated. On the other hand, the yeasts and algae required carbon dioxide and yielded copious amounts of oxygen as they grew. In Roger Hunter's little orbit-ship the ventilation shafts were small, a loose network of foot-square ducts leading from the central pumps and air-reconditioning units to every compartment in the ship. But in a ship of this size.... The grill was over a yard wide, four feet tall. It started about shoulder height and ran up to the overhead. The ducts would network the ship, opening into every compartment, and no one would ever open them unless something went wrong. And then he was laughing out loud, working the grill out of the slots that held it to the wall, trying to make his hands work in his excitement. He knew he had found his answer. The grill came loose, lifted down in a piece. He stopped short as footsteps approached in the corridor, paused, and went on. Then he peered into the black gaping hole behind the grill. It was big enough for a man to crawl in. He shinned up into the hole, and pulled the grill back into its slot behind him. Somewhere far away he heard a throbbing of giant pumps. There was a rush of cool fresh air past his cheek, cold when it contacted the sweat pouring down his forehead. He could not quite stand up, but there was plenty of room for him to crouch and move. Ahead of him was a black tunnel, broken only by a patch of light coming through the grill that opened into the next compartment. He started into the blackness, his heart racing. Somewhere in the ship Johnny Coombs and Greg Hunter were prisoners ... but now, Tom knew, there was a way to escape. * * * * *