His mind in an indescribable turmoil, Nelson splashed across a hundred yards of sodden snow, then shivered on wading knee deep through a pool of melted ice. Now he stood on the very threshold of that awful opening, dense clouds of vapor beating warmly against his chilled features.

His goggles fogged at once, blinding him effectively as, with reason staggering under the acc.u.mulated stress of starvation and the circ.u.mstances of Alden's disappearance, he groped his way a few feet into the vent. With his left hand he pulled up the gla.s.ses from his sunken, blood-shot eyes.

"It's warm, by G.o.d!" he cried in astonishment as the skin exposed by lifting the goggles came in contact with the air. "Must be some kind of earth-warmed cave."

Increasingly mystified, he caught up his rifle and strode on down the pa.s.sage, at that moment illuminated by the last unearthly rays of the aurora borealis. A single, dazzling beam played before him like a powerful searchlight, to light a high vaulted tunnel of basalt rocks which were distorted by some long-gone convulsion of the earth into a hundred weird cleavages and faults. For that brief instant he found he could see perhaps a hundred feet down into a high roofed pa.s.sage, along the top of which poured a tremendous stream of billowing, writhing steam.

"If this doesn't beat all," he murmured; but for all of his apprehension he did not pause. Those b.l.o.o.d.y splashes bespeaking Alden's pressing need urged him on. "Looks like I'm taking a one way trip into h.e.l.l itself. Well, we'll soon see."

Slipping and sliding over an almost impa.s.sable array of black rocks and boulders, Nelson fought his way forward, conscious that with every stride the air grew damper and warmer. Soon trickles of sweat were pouring down over his chest, tickling unbearably.

Then all at once the ray of light faded, leaving him immersed in a blackness equalled only by the gloom of a subterranean vault. He stopped and, resting his rifle against a nearby invisible rock, threw back the parka hood and pulled off his gloves. He was amazed to feel how warm the strong air current was on his hands.

"Beats all," he muttered heavily. "I wonder where they've taken Alden?"

Meanwhile his hands groped through fur garments now wet with melted-snow and ice particles, searching for the catch to open that pocket in which lay a small but powerful electric flashlight, an instrument without which no far-flying aviator finds himself. After a moment's fumbling, his yet stiffened fingers encountered the cylindrical flash and, with a low cry of satisfaction, he drew it forth to press the b.u.t.ton.

"Mighty useful. I--" The words stopped, frozen on his lips. Before the parka edge his close cropped hair seemed to rise, and his breath stopped midway in his lungs. Sharp electric shocks shook him, for there, half revealed in the feeble flashlight's glare, was a sight which shook his sanity to the snapping point. Not fifty feet away two eyes, large as dinner plates, with narrow vertical red irises, were trained on him. Rooted to the ground by the paralysis of utter horror, Nelson saw that their color was a weird, unhealthy, greenish white, rather like the color of a radio-light watch dial.

Strangely intense, these huge orbs wavered not at all, filling him with an unnameable dread, while the strong odor of musk a.s.sailed his nostrils. The flashlight slipped from between Nelson's fingers and, no longer having his thumb on the b.u.t.ton, flickered out.

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 s.n.a.t.c.h 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 t.i.tanic rat--squeaks that rang like the chorus of h.e.l.l 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."

Taking a fresh grip on his quivering nerves, Nelson cautiously c.o.c.ked the .38-55, cuddled the familiar stock to his shoulder. He sighted, then with his right hand pushed down the catch lever of the flashlight.

Instantly a dazzling white beam shot forth to shatter the gloom. The hair on the back of Nelson's hands itched unbearably, while the cold fingers of madness clutched at his brain, for the sight which met his eyes all but bereft him of his wavering sanity. There, belly up, across a low ridge of basalt, lay a hideous reptile, which in form faintly resembled an enormous and fantastic kangaroo. Its scabby belly was of the unhealthy yellow of a grub, a hue which gave way to a leaden gray as the wart-covered skin reached the back. Two enormous hind legs, each thick as a man's torso and each equipped with three dagger-like talons, struck out in helpless fury at the air, while a long, lizard-like tail threshed powerfully back and forth, scattering ponderous boulders right and left as though they had been marbles. The flashlight being trained as it was, the monster's head and forequarters were invisible, all save two very much smaller and shorter front legs which, like the hinder ones, clawed spasmodically.

"The D. T's!" gasped Nelson, conscious that he was trembling like an aspen. He suppressed a wild desire to laugh. "Yes, I've gone crazy!"

He glanced downwards and leaped swiftly back, for, creeping over the stones towards his fur outer boots, meandered a wide rivulet of bright scarlet blood. From its surface rose small curling feathers of steam which, drifting towards the tunnel's roof, merged with that gray, vaporous current flowing steadily towards the sunless Arctic expanse outside.

It took Nelson a long five minutes to sufficiently recover his equilibrium for action. All he could do was to stare at that grotesque, gargoyle-like creature as it writhed in leisurely and persistent death throes.

"Guess I winged it all right! My G.o.d, what a nasty beast! Looks like one of those allosaurs I read about in college. It couldn't be, though--that tribe of dinosaurs died out five million years ago."

Cautiously he scrambled around among the high black stones, casting the search light beams before him and holding the Winchester always ready in his hand while trying to recall s.n.a.t.c.hes of palaeontology studied at college long years ago.

"Yes, it must be a survival of one of the carnivorous dinosaurs," he decided, then paused, increasingly conscious of that steady thudding noise. What caused it?

At last he found himself before the creature's gigantic and repulsive head which lay limp over a blood bathed stone, huge jaws partially open, and serrated rows of wicked, stiletto-sharp teeth gleaming yellowly in the flashlight's rays. The head in shape was bullet-like, ending in a blunt nose as big as a bushel basket and in two prominent nostrils. The green, lidless eyes were still open, shining faintly, and seemed to follow his movements, but the steaming blood poured with the force of a small hose from between triple row of bayonetlike teeth that curved inward like those of a shark, to splash and bubble freely to the rock floor and to dribble horribly over the warty, gray hide.

Then Nelson discovered an amazing fact. About the great scaly neck, thick as a boy's waist, was fastened a ponderous collar, set with short, sharp spikes.

Nelson gasped. "What in h.e.l.l!" he cried. "This d.a.m.n thing's somebody's property!" His mind, staggered at the thought of dealing with a race that could and would domesticate such a hideous monster. "Well, it's no use standing here," he muttered, wiping the sweat from his eyes.

"This isn't getting poor Alden away from those devils."

_Thud! thud!_ In the act of turning he paused, listened once more.

Then he discovered to his amazement that the heart of the apparently dead reptile was still beating strongly. He could even see the yellow skin of its belly rise and fall. The effect was grotesque, uncanny.

"Of course," muttered the shaken aviator, "I'd forgotten a reptile's ganglions will keep on beating for hours, like that shark we killed off Paumotu. Its heart didn't stop for five hours."

Leaving the slain allosaurus behind, the aviator limped onwards, doggedly following a trail which wound down, ever onwards, into the depths of the earth. Gradually the air became so filled with steam that he stripped off his fur jumper and trousers. Clad in a khaki flannel shirt, serge trousers and shoepacks, he paused long enough to count his cartridges, and found there were just fourteen. h.e.l.l! Not very many with which to venture into an unknown abyss. He distributed them in his pockets, and, somewhat relieved of the weight of the fur suit, took up his advance, playing the flashlight ahead of him as he went.

"Poor Alden," he thought. "I wonder if he's still alive?"

Every moment expecting to stumble over the mangled corpse of his friend he hurried on, making better time over the cavern floor, but soon even the lighter clothing commenced to feel oppressive.

"Must be the earth's heat," he muttered, while the steam clouds rolled by him like ghostly serpents. "Guess the crust is very thin here--something like Yellowstone. Probably I'll find some thermal springs ahead."

Just as he spoke the tunnel took a sharp turn to the right. He scrambled around the bend to stand petrified, for with the suddenness of lightning a flood of dazzling orange-red light sprang into being.

Momentarily it blinded him, then revealed strange, incomprehensible scenes. It appeared that two short shafts of incandescent flame roared through transparent columns of gla.s.s on either side of the pa.s.sage some fifty yards distant. Subconsciously Nelson realized that these columns began and ended in stonework that was smooth and well joined.

As his eyes became accustomed to the glare he distinguished beside each light pillar two bronze doors, some eight feet high and semicircular in shape. These had been evidently pulled back to expose the lights. Then his breath stopped in his throat, for there, standing beside them, was a gleaming group of six or eight of the strangest creatures Nelson could ever have imagined. They were men--there was no mistaking that--men of normal size, but they were so helmeted and incased in a curious type of armor that for a moment he believed them gargoyles.