to the
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,
they saw a bare, unshaded lamp suspended from the roof of the tunnel.

Tom suddenly stopped and jerked Astro back. "Look!" he exclaimed,
pointing to the floor, not two paces away. A thin wire, hardly
noticeable, was stretched across the floor at ankle height.

"That bright light is to attract your attention while you trip over that
thing and probably blow yourself to bits," he said grimly, pointing to
the wall where the wire was connected to a small charge of explosives.
"Nothing to bring the roof down," he continued, "but enough to blast
whoever tripped over this wire."

Stepping over the wire carefully, they started down the shaft again, but
Tom paused thoughtfully.

"What's the matter?" asked Astro.

"That booby trap," said Tom. "We'd better not take any chances of
tripping over it on the way back. We might be in a hurry."

"I know what you mean," grunted the big Venusian. He knelt down beside
the menacing box of explosives and quickly disconnected the trip wire,
throwing the box to one side.

Straightening up, Astro announced, "It's harmless now."

Cautiously the two cadets continued down the tunnel, the roaring sound
growing louder and louder. After twenty minutes, Astro paused, his
homely features wrinkled in a fr