opening. Astro followed. Immediately below the trap, they found a
ladder, fixed to the wall of the shaft, which led directly down to a
point about thirty feet below the surface of Titan. At the bottom the
two cadets paused. A long tunnel stretched before them.

"Listen to that!" exclaimed Astro.

Tom ripped off the mask and listened. He heard a strange noise which
sounded more like the roar of escaping gas than a motor.

"What is it?" asked Tom.

"That's what I'd like to know!"

"And that light," continued Tom, pointing down the length of the tunnel.
"Do you suppose it's Miles and Brett?"

"It isn't moving," commented Astro.

"Well, since we're here we might as well find out as much as we can,"
Tom decided. "Let's go."

The two cadets flattened themselves against the side of the shaft and
inched forward. The hissing noise was slowly building up to a roar now,
and as they made their way along the shaft, they passed other smaller
tunnels that branched off to the left and right. There was evidence of
recent work. Tools were scattered along the tunnel floors, as if the
workers had dropped them in sudden flight.

The light ahead of them grew brighter, and as they rounded a corner,