own of worry.

"Think maybe Miles and Brett went off into one of the other side
tunnels?" he asked.

Tom thought a moment. "No, I don't, Astro. We haven't hit another side
tunnel since we passed that booby trap back there. What would be the use
of setting that thing up if they went in another direction?"

"There must be another way out of here, then," Astro remarked.

"Why?"

"That part of the tunnel back there by the bomb was loose dirt. If the
bomb had exploded, the whole tunnel would have been blocked off and how
could they get out?"

Tom didn't answer. He was thinking about what he was going to do if
there should be another booby trap in the tunnel. It was so dark now
that they could hardly see more than a few feet ahead. The bright light
was merely a pinpoint in the distance behind them.

And then Tom became aware that the roar that had been with them
constantly since they had entered the shaft had now lessened in volume.
But they had not passed a single branch-off tunnel where the sound could
have originated. Tom made up his mind quickly.

"Come on, Astro," he said. "We're going back."

"Why?"

"I haven't time to explain now, but you walk close to one side of the
shaft and I'll take the other. Feel along with your hands for anything
like a door or an opening. I think we've passed them."

Without another word, Astro turned around and headed back, feeling along
the tunnel wall.

It did not take the two cadets long to discover what they were looking
for. A heavy wooden door was set flush with the side of the tunnel. And
when Tom pressed his ear to it, he could hear the roaring sound
throbbing heavily inside.

"See if you can open it, Astro," said Tom. "But take it easy."

Astro felt along the side of the door until he found a wooden latch and
he lifted it gently. The door swung back, as if pushed, as a powerful
draft caught it from the other side. The roar was now deafening.

Tom stepped inside cautiously, followed by Astro. They found themselves
on a small balcony overlooking a huge subterranean room. In the room
they saw Quent Miles and Charles Brett bending over a table on which
were several delicate electronic instruments. Tom and Astro recognized
them immediately as testing machinery for radioactivity, much more
advanced and sensitive than the ordinary Geiger counter. Around the two
men was ample evidence of Astro's original assumption that they were
digging into a hot vein of uranium pitchble