"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