lows, and once or twice when the machinery was stopped the cadets
heard one man, who was evidently in charge, give a number of commands
in the German tongue.

For nearly an hour longer the pounding and clanking continued. Most of
the work was done on the rocky shore of the waterway, but some took
place inside the submarine and also on the forward deck of the craft.
The submersible was of good size, being over two hundred feet in
length.

"I'd like to get a whack at that U-boat," declared Gif. "I bet I'd
make it so she wouldn't do any more cruising in a hurry."

"Exactly what I was thinking, Gif," said Jack. "If we could only
injure the propellers, or something like that, there wouldn't be any
danger of their sailing away. You see, they may have it all fixed to
leave at a minute's notice in case of an alarm."

"If they did that they'd have to leave some of their machinery
behind," put in Fred. "But I suppose they'd be willing to do even
that rather than risk capture."

When the cadets thought that their ears could no longer bear the awful
noise to which they had been subjected, the sounds of the machinery
suddenly ceased. They heard a strange humming from the interior of the
submarine, but even this presently came to an end, and then there was
a silence within the cavern which was absolutely oppressive.

"I guess some kind of a move is at hand," whispered Jack. "Let's lay
low and see what they do next."

Several men, including two who were evidently officers and in charge
of the work, came out of the submarine. The workmen were now washing
up at the underground waterway, and presently all cast aside their
working clothes and donned ordinary street garments.

"Looks as if they were going to leave," whispered Fred excitedly.
"Maybe they are going out into the open for some fresh air. I can't
blame them for that," he added, for throughout the cavern there was a
strong smell of used gasoline.

Five minutes passed, and then one by one the Germans walked away from
the submarine. They did not come in the direction of the cadets, but
passed around another bend of the rocks, and so out of sight.

"That must be the way used by that fellow we saw in the forest,"
declared Jack.

All but three of the electric lights had been put out, so that the
interior of the cavern was now quite gloomy. The only sound that broke
the stillness was the soft lap, lap of some distant waves, evidently
where they broke on the shore of the