"There's no telling what Brett and Miles are liable to do while we're gone," said Tom. "You find Captain Strong and I'll go down into the shaft and look around."

"Not on your life," protested Astro. "You don't think I'd let you go down there alone, do you? _You_ go back to Captain Strong and _I'll_ see what those two are doing down there."

Tom grinned. "O.K., we'll both go down," he said.

Opening the trap door all the way, Tom eased himself down into 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 t.i.tan. 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 pa.s.sed 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 b.o.o.by 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 frown 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 pa.s.sed that b.o.o.by 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 b.o.o.by 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 pa.s.sed 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 pa.s.sed 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 a.s.sumption that they were digging into a hot vein of uranium pitchblende. To one side of the room, lead sheets lined a rough boxlike structure that Astro and Tom guessed was covering for the radioactive vein. Against the wall lay the lead-lined suits used by the miners. Further to one side, Tom saw a huge open pipe. He nudged Astro.

"Look, over there," Tom whispered. "That's where the oxygen is coming from!"

Below them, Miles suddenly walked to the pipe and pulled a large lever on its side. The roaring sound stopped immediately and the boys felt the air pressure in the room lessen slightly.

"That blasted noise is driving me crazy," explained Miles, walking back to the table, his voice echoing in the rock-walled cavern.

Brett, leaning over the table, was stabbing around futilely in one of the sets of tubes in a complicated testing device. "Wish we had that squirt Manning here," he mumbled. "He could fix these things up in no time at all."

"I could always go back to the hide-out and get him," suggested Miles.

On the balcony Tom gripped Astro's arm tightly.

"Astro! Did you hear that?" he exclaimed.

The big cadet nodded and started to rise from their place of concealment. Tom pulled him down. "Wait," he whispered sharply. "No use barging in on them yet. Maybe we can find out where Roger is first."

Astro reluctantly crouched down again, his hamlike hands balled into fists.

The two cadets watched Quent Miles and Brett work on the instruments awhile longer. Finally Miles slammed down a pair of wire cutters on the table and growled at Brett. "No use messing with this thing any longer.

I don't know what makes it tick, so I can't find the trouble. We need new equipment."

"It'll take at least two weeks to get new equipment the way things are going here at t.i.tan," replied Brett.