"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 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!"