Radium. Somewhere in this cavern was a deposit of radium, and Oscar had run into it. The hair of the _thlot_ glowed when in the presence of radio-active energy. Andy was laughing crazily. Radium ... more precious than diamonds. Fortune. The ship on Luna. His! He had forgotten they were locked in the cavern. "Come on," he yelped. "Lead us to the place where you started glowing." "It's back in there, in a ball. I don't want to go. Let's get out of here." "You take us there, or I'll break your damned neck." "Aw, Boss...." "Get going." Oscar, complaining bitterly, started off. They followed. The cave widened out into an immense chamber. In the center was a crucible of some kind, a cracked, battered crucible filled with glowing matter. Andy scratched his head, moved forward. "There it is, Boss, right there." A soft glow, like moonlight, filtered through the interstices of the crucible, dimly illuminating the cavern. Dust moved beneath their feet, dust that had not been disturbed for ages. Oscar sneezed. A heavy, cup-like crucible with cracked walls that had been several feet thick ... in the center was a softly glowing ball.... Andy bent over it.... Radium.... There was no doubt.... But.... "Hell," he said, his jaw dropping. "Hell...." His eyes caught the heavy outlines in the dust on the floor. He stirred it with his toe. "Intelligence," he muttered. "Intelligence was here, in this cavern perhaps a hundred centuries ago. The crucible is lead, incredibly old. Perhaps part of it was once radium. It was the heart of some kind of an engine, some method of releasing energy, possibly hundreds of thousands of years ago. Look! You can see in the dust where other metals, which formed a framework, have oxidized...." The girl said nothing, and Oscar, for once was silent. "Once there was intelligent life on Io. It built this, and left it for some reason that we can't even guess at." * * * * * Frieda stared at the glowing metal, moved back. "A fortune," she said. "Yours." "No," he corrected. "Ours." They were silent. The mighty cavern was silent. Dim ghosts seemed to move in it, the shadows of a mighty people that had once been here, and had gone.... "I want to get away from here," Oscar whimpered. "I don't like this place." Andy sighed. Their dust would mingle with the dust of the builders of the cavern. Another hundred thousand years would pass before the place was rediscovered. Maybe more.... "We can't," said Andy. "The entrance is blocked." "The hell we can't!" Oscar answered. "When all the shooting was going on the rocks started to fall in here, and I looked for a way out. The hill is hollow. There's an opening on the other side. Come on. Quit gaping at me, and get a move on." "_Thlot_," said Andy grimly. "If you're lying, I will break your neck." "I'm not lying. Come on. You can come back later. I itch from being near that shining stuff."