War Against the Rull by A. E. van Vogt

Barbara's leather-covered hands bit like sharp stones into his arm; he felt himself dragged over the edge.
    They landed unhurt among the loose pile of branches below and scrambled frantically to throw it farther down. A great mad clawing and horrible bass mewing above them whipped them to desperate speed. They made it just as that enormous head peered down from the second level, visible only by the phosphorescent glow of its eyes, like two burning coals a foot and a half apart.
    There was a terrific scrambling sound behind them as they pushed wildly down to the next level; a rock bounced down, narrowly missing them as it clattered past; and then, abruptly, silence and continuing darkness.
    "What's happened?" Jamieson asked in bewilderment.
    There was bitterness in her voice as she replied. "It's wedged itself in, because it's realized it can't get us in the few minutes left before it freezes for the night; and, of course, now we won't be able to get out past it, with that great body squeezed against the rock sides. It's really a very clever animal in its way. It never chases grasseaters but just follows them. It has discovered that it wakes up a few minutes before they do; naturally, it thinks we, too, will freeze and that it will wake up before we will. In any event, it knows we can't get out. And we can't. We're finished."
    All that long night Jamieson waited and watched. There were times when he dozed, and there were times when he thought he was dozing, only to realize with a dreadful start that the horrible darkness had played devil's tricks on his mind.
    The darkness during the early part of the night was like a weight that held them down. Not the faintest glimmer of natural light penetrated that Stygian night. And when, at last, they made a fire from their pile of brush, the pale, flickering flames pushed but feebly against the pressing, relentless force of the darkness and seemed helpless against the cold.
    Jamieson began to notice the cold, first as an uncomfortable chill that ate into his flesh, and then as a steady, almost painful, clamminess that struck into his very bones. The cold was noticeable, too, in the way white hoarfrost thickened on the walls. Great cracks appeared in the rock; and not once but several times sections of the ceiling collapsed with a roar that threatened their lives. The first clatter of falling debris seemed to waken the woman from a state of semi-coma. She staggered to her feet; and Jamieson watched her silently as she paced restlessly to and fro, clapping her gloved, heated hands together to keep them warm.
    "Why not," Jamieson asked, "go up and build a fire against the gryb's body? If we could burn him—"
    "He'd just wake up," she said tersely, "and besides, his hide won't burn at ordinary temperatures. It has all the properties of metallic asbestos—conducts heat but is practically noncom-bustible."
    Jamieson was silent, frowning; then he said, "The toughness of this creature is no joke—and the worst of it all is that our danger, the whole affair, has been utterly useless. I'm the only person who has a solution to the ezwal problem, and I'm the one you're trying to kill."
    "I don't really suppose it matters," she said. "What's the use of you and I arguing on this subject? It's too late. In a few hours that damn thing that's got us sealed in here will wake up and finish us. There's nothing we've got that can hold it back one inch or one second."
    "Don't be so sure of that!" Jamieson said. "I admit the toughness of this monster has me worried, but don't forget what I've said: these problems have been solved before on other planets."
    "You're mad! Even with a blaster, it's touch and go getting
    the gryb before it gets you. Its hide is so tough it won't begin to disintegrate until your heart's in your boots. What can we do against a thing like that when all we've got is a knife?"
    "Let me have the knife," Jamieson replied. "I want to sharpen it" His face twisted into a wry smile.