yelled. "Now--down once more!"

The rockets were slowly cut; the borer jarred at the bottom of its
hole; again the disintegrators droned out. The sphere dug rapidly into
the warm ground, biting lower and lower. At ten miles an hour it
blasted a path to depths hitherto unattainable to man, sweeping away
rock and gravel and sand--everything that stood in its way. The depth
gauge rose to two thousand, then steadily to three and four. So it
went on for nearly half an hour.

At the end of that time, at a depth of nearly four miles, Quade got
stiffly to his feet and once more shouted into the professor's ear.

"We ought to be close to that radium, now," he said. "I think--"

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 muffled 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