But his words stopped short. The floor of the sphere suddenly fell away from their feet, and they felt themselves tumbled into a wild plunge. The drone of the disintegrators, hitherto m.u.f.fled by the earth they bit into, rose to a hollow scream. Before the professor quite knew what was happening, there was a stunning crash, a shriek of tortured metal--and the earth-borer rocked and lay still....

The whole world seemed to be filled with thunder when David Guinness came back to consciousness. He opened his eyes and stared up into a darkness to which it took him some time to accustom himself. When he did, he made out hazily that he was lying on the floor of a vast dark cavern. He could dimly see its jagged roof, perhaps fifty feet above.

There was the strong smell of damp earth in his nostrils; his head was splitting from the steady drone in his ear-drums. Suddenly he remembered what had happened. He groaned slightly and tried to sit up.

But he could not. His arms and legs were tied. Someone had removed him from the earth-borer and bound him on the floor of the cavern they had plunged into.

David Guinness strained at the rope. It was futile, but in doing so he twisted his head around and saw another form, similarly tied, lying close to him. He gave a little cry of relief. It was Sue. And she was conscious, her eyes on his face.

She spoke to him, but he could not understand her for the drone in his ears, and when he spoke to her it was the same. But the professor did not just then continue his effort to converse with her. His attention was drawn to the borer, now dimly illuminated by its portable light, which had been secured to the door. It was right side up, and appeared to be undamaged. The broad ray of the searchlight fell far away on one of the cavern's rough walls. He could just make out James Quade standing there, his back towards them.

He was hacking at the wall with a pick. Presently he dropped the tool and wrenched at the rock with bare hands. A large chunk came loose. He hugged it to him and turned and strode back towards the two on the floor, and as he drew near they could plainly see a gleam of triumph in his eyes.

"You know what this is?" he shouted. Guinness could only faintly hear him. "Wealth! Millions! Of course we always knew the radium was here, but this is the proof. And now we've a way of getting it out--thanks to your borer! All the credit is yours, Professor Guinness! You shall have the credit, and I'll have the money."

Guinness tugged furiously at his bonds again. "You--you--" he gasped.

"How dare you tie us this way! Release us at once! What do you mean by it?"

Quade smiled unpleasantly. "You're very stupid, Guinness. Haven't you guessed by now what I'm going to do?" He paused, as if waiting for an answer, and the smile on his face gave way to a look of savage menace.

For the first time his bitter feelings came to the surface.

"Have you forgotten how close I came to going to jail over those charges of yours a year ago?" he said. "Have you forgotten the disgrace to me that followed?--the stigma that forced me to disappear for months? You fool, do you think I've forgotten?--or that I'd let you--"

"Quade," interrupted the older man, "you know very well you were guilty. I caught you red-handed. You didn't fool anyone--except the jury that let you go. So save your breath, and, if you've the sense you were born with, release my daughter and me. Why, you're crazy!" he cried with mounting anger. "You can't get away with this! I'll have you in jail within forty-eight hours, once I get back to the surface!"

With an effort Quade controlled his feelings and a.s.sumed his oily, sarcastic manner. "That's just it," he said: "'once you get back!' How stupid you are! You don't seem to realize that you're not going back to the surface. You and your daughter."

Sue gasped, and her father's eyes went wide. There was a tense silence.

"You wouldn't dare!" the inventor cried finally. "You wouldn't dare!"

"It's rather large, this cavern," Quade went on. "You'll have plenty of room. Perhaps I'll untie you before I go back up, so--"

"You can't get away with it!" shouted the old man, tremendously excited. "Why, you can't, possibly! Philip Holmes'll track you down--he'll tell the police--he'll rescue us! And then--"

Quade smiled suavely. "Oh, no, he won't. Perhaps you remember the shot that sounded from the water-hole? Well, when I and my a.s.sistant, Juan, heard Holmes say he was going for water, I told Juan to follow him to the water-hole and bind him, to keep him from interfering till I got back up. But Mr. Holmes is evidently of an impulsive disposition, and must have caused trouble. Juan, too, is impulsive; he is a Mexican.

And he had a gun. I'm afraid he was forced to use it.... I am quite sure Philip Holmes will not, as you say, track me down."

David Guinness looked at his daughter's white face and horror-filled eyes and suddenly crumpled. Humbly, pa.s.sionately, he begged Quade to take her back up. "Why, she's never done anything to you, Quade!" he pleaded. "You can't take her life like that! Please! Leave me, if you must, but not her! You can't--"

But suddenly the old man noticed that Quade was not listening. His head was tilted to one side as if he was straining to hear something else. Guinness was held silent for a moment by the puzzled look on the other's face and the strange way he was acting.

"Do you hear it?" Quade asked at last; and without waiting for an answer, he knelt down and put his ear to the ground. When he rose his face was savage, and he cursed under his breath.

"Why, it's a humming!" muttered Professor Guinness. "And it's getting louder!"

"It sounds like another borer!" ventured Sue.

The humming grew in volume. Then, from the ceiling, a rock dropped.

They were looking at the cavern roof and saw it start, but they did not hear it strike, for the ever-growing humming echoed loudly through the cavern. They saw another rock fall; and another.

"For G.o.d's sake, what is it?" cried Guinness.

Quade looked at him and slowly drew out his automatic.

"Another earth-borer, I think," he answered. "And I rather expect it contains your young friend Mr. Holmes. Yes--coming to rescue you."

For a moment Guinness and his daughter were too astounded to do anything but gape. She finally exclaimed:

"But--but then Phil's alive?"

James Quade smiled. "Probably--for the moment. But don't let your hopes rise too high. The borer he's in isn't strong enough to survive a fifty-foot plunge." He was shouting now, so loud was the thunder from above. "And," he added, "I'm afraid he's not strong enough to survive it, either!"

CHAPTER II

_The Man-Hunt_

When Phil Holmes started off to the water-hole, his head was full of the earth-borer and the imminent descent. Now that the long-awaited time had come, he was at fever-pitch to be off, and it did not take him long to cover the mile of sandy waste. His thoughts were far inside the earth as he dipped the jug into the clear cool water and sloshed it full.

So the rope that snaked softly through the air and dropped in a loop over his shoulders came as a stark surprise. Before he knew what was happening it had slithered down over his arms and drawn taut just above the elbows, and he was yanked powerfully backwards and almost fell.

But he managed to keep his feet as he staggered backward, and turning his head he saw the small dark figure of his aggressor some fifteen feet away, keeping tight the slack.

Phil's surprise turned to sudden fury and he completely lost his head.

What he did was rash; mad; and yet, as it turned out, it was the only thing that could have saved him. Instinctively, without hesitating one second, and absolutely ignoring an excited command to stand still, he squirmed face-on to his aggressor, lowered his head and charged.

The distance was short. Halfway across it, a gun barked, and he heard the bullet crack into the water jug, which he was still holding in front of himself. And even before the splintered fragments reached the ground he had crashed into the firer.

He hit him with all the force of a tackling lineman, and they both went down. The man grunted as the wind was jarred out of him, but he wriggled like an eel and managed to worm aside and bring up his gun.

Then there was a desperate flurry of bodies in the coa.r.s.e sand. Holmes dived frantically for the gun hand and caught it; but, handicapped as he was by the rope, he could not hold it. Slowly its muzzle bent upward to firing position.

Desperately, he wrenched the arm upwards, in the direction it had been straining to go, and the sudden unexpected jerk doubled the man's arm and brought the weapon across his chest. For a moment there was a test of strength as Phil lay chest to chest over his opponent, the gun blocked between. Then the other grunted; squirmed violently--and there was a m.u.f.fled explosion.

A cry of pain cut the midnight air, and with insane strength Holmes'

ambusher fought free from his grip, staggered to his feet and went reeling away. Phil tore loose from the rope and bounded after him, never feeling, at the moment, his powder-burned chest.

And then he halted in his tracks.

A great roar came thundering over the desert!