The amazing crab-thing wore a metal harness. Bands of silvery aluminum were fastened about its shell, with little cases of white metal dangling to them. In one of its uplifted claws it carried what seemed to be an aluminum bar, two feet long and an inch thick. It scraped lumberingly past, between us and the racing stream of white fire. It passed less than a dozen feet from us. The curious fishy smell of it was overpowering, disgusting. Sweat of horror chilled my limbs. The monster emanated power, sinister, malevolent power, power intelligent, alien and hostile to man. I trembled with the fear that it would see us, but it scrambled grotesquely on. When it was twenty yards past, Ray picked up a block of black lava that lay beneath his hand and hurled it silently and swiftly. It crashed splinteringly on the rocks far beyond the creature, on the other side of the stream of light. In fascination I watched the monster as it paused as if astonished. The glittering compound eyes twisted about on their stalks, and the long shining green tentacles wavered questioningly. Then the knobbed limbs snapped the white metal tube to a level position. A metallic click came from it. And a ray of red light, vivid and intense, burst from the tube. It flashed across the river of fire. With a dull, thudding burst it struck the rocks where the stone had fallen. It must have been a ray of concentrated heat. Rocks beneath it flashed into sudden incandescence, splintered and cracked, flowed in molten streams. * * * * * In a moment the intensely brilliant ruby ray flashed off. The rocks in the circle where it had struck faded to a dull red and then to blackness, still cracking and crumbling. To my intense relief, the monstrous crab lumbered on. "That," Ray whispered, "is what got Major Meriden's airplane wing." When we could hear its scraping progress no longer, we climbed up from behind our boulder and continued cautiously down the cavern, beside the rushing luminous river. In half a mile we came to a bend. Rounding it, we gazed upon a remarkable sight. We looked into a huge cavity in the heart of the earth. A vast underground plain lay before us, with the black lava of the roof arching above it. It must have been miles across, though we had no way to measure it, and it stretched down into dim hazy distance. Its level was hundreds of feet below us.