e impeding
stalagmites, surrounded by a dark blur of shadows, the ghostly
phosphorescent light illuminating well only the few rods around them.
Utter silence brooded over the tunnel.

Phil paused when they had gone about seventy-five feet. "I left that
rope behind," he said, "and we may need it. I'll return and get it,
and you both wait right here." With the words he turned and went back
into the shadows.

He went as fast as he could, not liking to leave the other two alone.
But when he had retrieved the rope and tied it to his waist, he
permitted himself a last look up as he passed under the hole in the
ceiling--and what he saw there tensed every muscle in his body, and
made his heart beat like mad. Again there was a tiny spot of orange in
the blackness above!

"Professor!" he yelled excitedly. "Sue! Come here! The sphere's
coming back!"

There was no doubt about it. The pin-point of light was growing each
second, with the flame of the descending exhausts. Guinness and his
daughter ran from the tunnel, and, guided by Phil's excited
ejaculations, hurried to his side. Their eyes confirmed what his had
seen. The earth-borer was coming down!

"But," Guinness said bewilderedly, "those rockets were enough to lift
him!"

This was a mystery. Even though ten rockets were on--ten tiny spots of
orange flame--the sphere came down swiftly. The same force which some
time before had lifted it slowly up was now insufficient. The roar of
the tubes rose rapidly. "Get back!" Phil ordered, remembering the
danger, and they all retreated to the mouth of the tunnel, ready to
peep cautiously around the edge. Holmes' jaws were locked tight with
grim resolution. Quade was coming back! he told himself exultantly.
This time he must not go up alone! This time--!

But his half-formed resolutions were idle. He could not know what
frightful thing was bringing Quade down--what frightful experience was
in store for them all....

CHAPTER IV

_Spawn of the Cavern_

In a crescendo of noise that stunned their ears, the earth-borer came
down. Tongues of fire flared from the hole, speared to the ground and
were deflected upward, cradling the metal ball in a wave of flame.
Through this fiery curtain the machine slowly lowered to the floor,
where a shower of sparks spattered out, blinding the eyes of the
watchers with their brilliance. For a full minute the orange-glowing
sphere lay there, quivering from the vibration; then the exhausts died