Helpless, Nelson stood transfixed against a boulder, aware that the strange, musky scent was becoming stronger. Then to his ears came a dry scrabbling as of some large body stealthily advancing. Those horrible, unearthly eyes were coming nearer! Fierce, terrible shocks of fear gripped the exhausted aviator. Then the impulse of self-preservation, that most elementary of all instincts, forced him to snatch up the rifle, to sight hastily, blindly, between those two, great greenish eyes. Choking out a strangled sob of desperation, Nelson made his trembling finger close over the cold strip of steel that must be the trigger. * * * * * Like a stage trick, the cavern was momentarily lit by a strong, orange yellow glare. Then the Winchester's report thundered and roared deafeningly; coincidentally arose a nerve-shattering scream. An exhalation, foul as a corpse long unburied, fanned his face. Terrified, he flattened to the rock wall as a huge, though dangerously agile body hurtled by with the speed of a runaway horse. Presently followed the sound of a ponderous fall, then a series of shrill, ear-piercing gibberings and squeakings, like those of a titanic rat--squeaks that rang like the chorus of Hell itself. Gradually they grew fainter, while in the darkness the heavy air of the tunnel became rank with the odor of clotting blood. Nelson remained where he was, shaking like a frightened horse and bathed with a cold sweat. "Wonder what it was?" he muttered numbly. He broke off, for in the terrible darkness sounded a low but perfectly audible _thud! thud! thud! thud!_--and also the subtle noise of some rough surface rasping gently over the stone. His nerves crisped and shrieked for relief. "It's coming again!" he told himself, and ejected the spent cartridge from the Winchester. "No use--it'll get me, but I may as well fight as long as I can." Even stronger grew the musty smell of blood while that uncanny _thud! thud!_ sound continued at regular intervals. Nelson waited, breath halted and finger on trigger, but still the darkness yielded no glimpse of those awful saucer-like eyes. * * * * * Emboldened, he stooped and, jerking off his left glove, commenced to grope among the boulders. Somewhere near at his feet the flashlight must be lying. Hoping against hope that its fall had not shattered the bulb, he ran his fingers over the cold, damp stones, every instant expecting to feel the clutch of the unseen monster. How tiny, how puny he was! All at once his fingers encountered the smooth familiar shape of the flash and he raised it cautiously through the darkness. Patiently he shifted the Winchester to his left hand in order to set the flashlight on the top of a flat rock, pointing it as nearly as he could determine in the direction from whence came those ominous, stealthy sounds. "Guess I'll switch on the light," he decided, "and trust to drop whatever it is before it reaches me."