asked Tom.

Astro spun around and began to examine the walls. "Just as I thought!"
he exclaimed. "This room is airtight! Sealed! Oxygen is being pumped in
here."

"From where?"

"Might be from somewhere below," replied the big Venusian. "Down that
trap where Miles and Brett went."

Tom put his mask back on and headed for the door. Astro followed him.
They opened it a little and peered into the swirling mist.

"Then it's being pumped in directly," Tom asserted. "Through a duct
leading directly up into this room from somewhere below."

Astro nodded. "Then there's only one thing left to do. Go down through
that trap door and see what we can find." He stepped forward.

"Wait a minute, Astro," said Tom, stopping him. "Let me check our
oxygen. There might not be any down there. Remember, Miles and Brett
wore _their_ masks."

Making a quick check of their oxygen supply, Tom patted Astro on the
back and started forward. "It's O.K. We've got another four hours left.
Come on!"

They moved toward the trap door slowly.

"I still wish I had a ray gun," whispered Tom.

"As long as I can use these"--Astro balled his hamlike hands into
fists--"we're O.K."

When they reached the trap door, Tom got down on his knees and felt
around for the opening. He found a small ring bolt, motioned to Astro to
step back, and pulled. The trap door swung back easily and a shaft of
white light gleamed in his face. The young cadet leaned down and looked
through the opening. What he saw made him gasp.

"What is it?" demanded Astro.

[Illustration: _Tom got down on his knees and felt around for an
opening_]

Tom motioned for him to get down and look. The big cadet dropped lightly
to his knees to peer through the opening. "By the moons of Jupiter," he
exclaimed, "it's a--a mining shaft!"

"Just what we thought it was," whispered Tom. "Come on. Let's go down
and find out where it leads."

"Maybe we'd better go back and tell Captain Strong about this first,"
Astro said speculatively.

"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 in