Both men were talking vaguely, thinking vaguely, looking around vaguely. In part this must have been because of sheer wonder. Places like the Spacemen's Haven on Ganymede seemed as far away as a dream now. An incomprehensible sense of depression was creeping over Evan Harwich, as he studied his surroundings further. There were many other cages in view, arranged in blocks, with paved alleyways between. Vegetation was thick in the evidently air-conditioned habitations. Little pools of water glistened in them daintily, strange paradox on dying Io. And there were creatures, too. Scores of them in each cage. Strange, fragile, sluglike animals crept about aimlessly. They looked just faintly human, with their pinkish skins and manlike heads. But there was no slight shadow of intelligence in those great, sad, stupid eyes. Harwich wasn't squeamish, but he looked at these futile animals with a certain pitying revulsion. "What kind of a nursery place have we got ourselves into, Paul?" he grumbled quizzically. Arnold shrugged. "They're something like men, these things, aren't they?" he offered in puzzlement. "Maybe that's another unknown quantity to figure out. But this place is plenty wonderful, though. Look!" The youth was pointing upward. Against the cold Ionian sky a flattened object was circling at low altitude. A flying machine without wings, it seemed to be. From it dangled strange webby metal arms, as it moved in a circular path, above the surrounding desert hills. It seemed to keep watch over those thousands of crystal cages in the valley. It must be a guardian of some sort. "I'm not at all sure I like it here," Harwich growled. "We were fixed up, revived, made new men again, so to speak; but still I don't like it here." "Somehow I've got the same idea," Paul Arnold agreed with a quizzical smile. A little clinking noise behind the two men made them turn about. After that, awe kept them spellbound. They didn't speak. What was there to say? They didn't try to retreat, either. What was the use? If what they saw was danger, they could do nothing to avert it. Hypnotized with wonder, they only stared, feeling as helpless as the larvae in an ant-hill, tended and cared for by the workers. * * * * * A section of the cage-bottom had raised, like a trapdoor. A bulk was creeping through the opening. It was a machine, so marvelous, so refined in its functioning, that it seemed far more than alive. It was flat, like a small tractor; but there were no treads for it to move on. It seemed, rather, to glide on a cushioning, grayish mist. The thing purred softly, like a great cat, and tiny lights twinkled in crystalline parts of it--batteries to deliver fearful atomic or cosmic power, perhaps. The mechanism had many flexible tentacular arms of metal that glinted with a lavendar luster.