"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