"Hard going, Keith. G.o.d--the cavern's right ahead!"

It was ghostly to hear Hemmy's warm voice from the lifeless solitude outside. Breath coming quickly, Wells watched the silent scene--the cleft in the wall of rock overshadowing everything now. The diver fought ahead, gaining inch by inch.

Now, save for occasional clumps of weed, he was exposed to the enemy.... Now the last desperate gauntlet was reached.... Keith felt his blood pound hotly.

"I'm gaining, Keith. Gaining...."

Bowman had little breath for speech. His tiny form battled on, now sinking from sight as he dropped into some masked gully, now wrestling slowly with great swaying strands of kelp, but always struggling ahead.

"I'm at the bow, Keith! The hawser arm's right in our mooring holes.

I'll go halfway before fastening the charge. Any signs of life from the devil?"

"None yet, Hemmy. But go slow. Hide all you can, old man, for G.o.d's sake!..."

Right beneath the metal arm, Bowman's dwarfed figure crept doggedly ahead. Forward, inch by breathless inch. Kelp thickened, washed away; the two hulking submersibles, captor and captive, surged onward--but just a little faster went the valiant figure with the black charge on its back.

The towing monster had its snout in the cavern. The darkness thickened. Bowman was quarter way!

He plunged desperately. Half way!

"I'm there, Keith! Now for it!"

"Oh, G.o.d!" Wells cried. "They see you; they're coming!"

For he had seen strange shapes leaving the enemy submarine.

And at that same moment, Bowman saw them, too.

They came like the blink of a dark eye from a door that had quickly slid open in the mysterious ship's bow. As tall as a man they were, and there were two of them, though at first the nature of their bodies merged with the wreathing kelp made them seem like a dozen.

Bowman stared at them, hypnotized with fear. His legs and arms went dead, and his whole gallant spirit seemed to slump into lifeless clay.

Now he knew why the fishermen had shrieked "machine-fish." Each one of them had eight tapering arms, eight restless tentacles. These were octopi, most hideous scavengers of the ocean floor! And not only octopi--but octopi sheathed in metal-scaled armor!

As they came closer, he realized this preposterous fact. The dark substance of their writhing tentacles was not flesh: it was a coat of metal scales. And the fat central ma.s.s which held their eyes and vital organs and beaked jaw--this ma.s.s was completely enveloped by a globe of gla.s.s. From inside, he could see great eyes staring at him. The monsters came towards him quite slowly, obviously wary, advancing over the sea-floor in what was a hideous mockery of walking, their forward tentacles outstretched.

With a sob, Hemmy Bowman pulled himself from his trance. He glanced back at the _NX-1_. He still had time to retreat. He might be able to get back inside before these monsters seized him.

But that meant abandoning his job. And already his own submarine was nosing into the cavern. The choice between the octopi and retreat stared him in the face. He pulled himself together and jerked his arms back to action.

Eyes bulging, Keith Wells peered at the dim teleview screen. He saw the creatures approaching Hemmy. And then, suddenly, he remembered his radiophone.

"Hemmy! Come back, for G.o.d's sake!" he cried. "Come back while you can--it's hopeless!"

But Bowman had already seized the depth charge from his back and hooked it on the hawser arm above.

Immediately, with that action, all caution fled from the approaching monsters. Their tentacles whipped furiously; and in a great arc they sprang for the tiny figure of the diver.

With a deep breath, Hemmy staggered forward to meet them. "Keith!" he gasped. "I'll try to hold 'em away from the charge! When it bursts, zoom! Zoom like h.e.l.l to the surface!" And then the tentacles had him.

Keith watched, cursing his impotence to help. Hemmy had no weapon; he was trying to hold them back by the weight of his body; he reached out and grasped a tentacle and hugged it to him, shoving forward with all his puny strength. But all his effort was as nothing. One of the octopi writhed past him and darted onto the depth charge. Its tentacles tugged at the bomb; pulled furiously.

The time charge exploded. The _NX-1_ rocked like a quivering reed; Wells was knocked violently to the floor; a vast roar smote his ear-drums. When he staggered to his feet he found that the octopus that was pulling at the charge had disappeared--blown into fragments of flesh and metal. But the hawser arm was broken! The _NX-1_, free, shot back a full fifty feet under the pull of her reversed screws. A cry echoed in her commander's ears:

"Go back, Keith! Go like h.e.l.l!"

He saw the remaining octopus lift Bowman and whip to the exit port of its submarine. The lid slid into place, closing on the monster and his friend, and the enemy ship vanished into the black cavern....

Once clear of the opening, Keith set his motors full forward and brought the diving rudders up. Quickly the ship sped from the haunted sea-floor to the sun-warmed surface. A last thin call rang in his radiophone:

"They've got me inside, Keith. It's dark, and filled with water. I can't see anything, but I--I guess we're going through the cavern....

Forget about me, old boy. So long! So--"

The voice was abruptly cut off.

Keith ripped the instrument from his head. Then, face white and drawn, he ran to the radio cubby. Standing over Sparks' inert body, he put through a call to Robert Knapp, on the _Falcon_.

"Knapp?" he said harshly. "This is Wells. I'll be with you in a few minutes. Yes--yes--I'll tell you the whole story later. But get this now: Have the day shift all ready to take over the submarine by the time I pull alongside."

He said no more just then; but rang off, and, looking back, he muttered savagely:

"But I'll be back, Hemmy--I'll be back!"

CHAPTER IV

_In the Cavern_

"That's the story, Knapp. They got Bowman, and I had to run away.

Their ship disappeared into the cavern. I've got a hunch, though, that it's not just a cavern, but a tunnel, leading through to some underwater world. That series of sub-sea earthquakes probably opened it up; and now these devil-octopi are free to pour out. I've _got_ to find out what's what, and that's why I'm going down again as soon as the torpedo system's ready!"

Keith and Robert Knapp were in the _Falcon's_ chart room. On the table before them lay a broad white map with a cross-mark indicating the position of the mysterious dark cavern.

Wells was striding up and down like a caged tiger in his impatience to be off. Every other minute he glared down to where the _NX-1_ lay alongside. On her conning tower stood the tall blond-haired figure of Graham, the first officer of the day shift, supervising the final details of the work of installing a system of jury controls whereby the submarine's torpedoes could be fired from her control room.

Keith stopped short and faced Knapp. "It won't be so one-sided this time, Bob," he promised. "You see: when the location chart shows the enemy ship, I'll rush all men into the control room, where the paralyzing ray can't harm them. I don't know but what they have in other weapons, but I'm gambling on getting my torps in first. They've killed Bowman; they've ravaged a whole fishing fleet; they're free to emerge from their hole and maraud every ocean on the globe! They've got to be stopped! And since I'm armed and have the only submarine on the spot, I've got to do it! I know how to fight them now!"

Captain Robert Knapp's sense of things was badly disordered. He had just heard a story which his common sense told him couldn't be true, but which the evidence of his eyes had grimly authenticated. He had seen fifteen men slung aboard his ship from the _NX-1's_ silent hull; men stretched in grotesque, limp att.i.tudes; men struck down by a paralyzing ray. Why, no nation on earth had developed rays for warfare! Yet--a crew of helpless men was even then in the sick bay, receiving attention in the hope that they might recover.

"You're going right through that cavern, then, Wells?" he asked incredulously. "You're going to investigate what lies beyond?"

"Nothing else! And I won't come out till I've blown that octopi ship to pieces!"