hin a stone's throw of it.

Twice they paused. A silence so deep that it was painful ensued. No sound
came. They marched solemnly on. And now, they had struck a steep incline.

"Down low; down low; down low," came whispered back from man to man.

They stooped to an almost creeping posture and began to climb. The ascent
was steep as a stair. Twice Dave lost his footing, and once came near
sending his rifle crashing to the frozen earth. Some one behind was less
fortunate. There came the clang of steel, then deathly silence.

Again they crept upward. Suddenly a ray of light cut through the gloom. In
another second, they were in a veritable flood of light. And yet, as they
glanced rapidly to right and left, they saw walls of rock. Above them too
was a vaulted ceiling. Only before them was light. What could it mean?

In an instant they knew. Leaping toward the opening, they expressed their
surprise in unchecked exclamations.

"A balloon! A balloon!"

It was true. It seemed to them, as they looked, that the whole side of the
mountain had burst open and allowed a giant dirigible balloon to float out
from its depth.

What had really happened was evident. These robbers, having located the
rich mine and having no concession to mine it, had discovered this natural
cave and had cut a channel from it to the place of the gold deposit. They
had reached the point by balloon. Having deflated it, they had stowed it
away in the cave and had blocked the entrance of the cave with snow. The
next blizzard had defaced every sign of their presence. Doubtless there
had been a small secret entrance to the cave which none of Johnny
Thompson's men had discovered.

"They're gone!" exclaimed Dave.

"And I 'ates to think 'ow much gold they took with 'em," mourned Jarvis.

"Quick, the airplane!" shouted Pant, turning to the two aviators. "There's
a machine gun on it. We'll halt them yet. I better go with you. Some of
the rest of you explore the interior here. They may not have taken the
gold."

Dave Tower snapped on his flashlight, and, after taking one more look at
the giant black "sausage" in the sky, turned to assist the others in the
exploration of the looted mine. He had little hope of discovering the
treasure, but he did want to see how they had accomplished the task.

One more question crowded its way to the front: "How had they killed Frank
Langlois?"

CHAPTER VIII

THREE MEN DISAPPEAR

It was the dirigible balloon th