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 into the