"That's a darned good idea," Andy answered. "We better move out of here, and move fast." Frieda needed no urging. Her face was white, but she held her heat-gun resolutely as they skipped over the rocks. Andy had his blaster out, and was searching vainly for a target. Another explosion shook the ancient, time-worn hills. Andy snapped three shots at random and three explosions followed. This business was a game that two could play at. [Illustration: _Andy's blaster roared again. "Quick!" he barked, "Get inside the cave. It's our only chance...."_] "This way," Frieda panted, the thin air of Io not providing enough oxygen for fast running. * * * * * Following her pointing finger, Andy saw a dark opening yawning in the face of the bluff. In other circumstances he would have instantly noticed that it had an artificial appearance, as if the cave had been cut into the stone by other than natural means. Frieda came to a panting halt just inside the entrance, but Oscar, his tail between his legs, skipped rapidly out of sight into the dark cavern. The _thlot_ loved peace. "Frieda--Miss Dahlem, I mean," Andy panted. "We're safe here, at any rate." Almost immediately a blast shook the cavern. Loosened stone fell from the roof, there was a shower of debris outside, the cavern rumbled, and the light coming in through the entrance faded as a landslide almost completely blocked the opening. "Hell!" Andy gasped, "another shot like that and we'll be buried alive. This is no place for us. Let's get out of here." He moved to the entrance, his earth-sired muscles thrusting aside slabs of stone that he could not have handled on earth. Frieda worked with him. Together they cleared a space of less than two feet at the top of the cavern, which would allow them to slip out. Andy stuck his head out and immediately jerked it back. "There are three of 'em. They're on top of the hill and they've got a blaster trained on us. Luckily they didn't see me, but if we try to run, they'll blow us to smithereens." "Can't you get a clear shot at them?" "Maybe. But if I miss, they'll blow enough junk over the mouth of this cave to bury us a mile deep. Too much chance. What's eating them, anyhow?" "They saw some very rich samples that I had dug out of the quartz vein I told you about. If the whole vein were as rich as those samples, it would be worth a fortune, and they think it is that rich. Having tried to take it, they know they have to kill us, for if we escape, the space police will round them up and give them a shot of gas." "Um--I see. Looks tough on the home team." She didn't answer. Andy cautiously stuck his head outside and jerked it back as another atomic pellet dislodged a huge stone that came sliding down the hill. "Did they see you?"