bay close to the larger entrance
of the cavern.

"I'd like to bet that the entrance is under water," said Fred. "If it
was at the surface some one would have discovered this place a long
time ago." And in this surmise the youngest Rover was correct. The
passageway, which was amply large, was over ten feet below the surface
of the bay even at low tide.

"Do you suppose they've all left the submarine?" whispered Gif
presently. With the intense silence prevailing, they felt that they
must be very cautious in making any noise.

"That's hard to say," answered Jack, with a shrug of his shoulders.

"It looks so to me," put in Fred. "I don't believe any of those
fellows would care to stay down here unless it was necessary. They
have stopped all the engines and things like that. I guess those
electric lights are burning simply from a storage battery."

The three cadets waited for another ten minutes, and then, as no one
appeared, and as the submarine seemed to be deserted, they stole
forward cautiously, all anxious to get a closer look at the U-boat.

"If we could only throw a chain around the propellers, or something
like that, maybe it would keep them from getting away if they tried to
run for it," said the young captain.

"We'll look around and see what we can do, anyway," answered his
cousin.

"There are plenty of chains around," put in Gif. "Those are what made
the awful clanking sounds we heard."

Step by step the three cadets came up until they were at the spot
where the Germans had set up their repair plant. Some of the things
they had been working upon were still lying about, but other parts had
been taken aboard the submarine.

"I guess they have all gone," said Jack, after a look around. "I'm
going aboard that craft and take a peep at her."

The others were also anxious to do this, and all three were soon
across the gangplank which led to the open hatch of the U-boat. They
gazed down this hatch with some awe, and discovered that several
electric lights had been left turned on below. A steel ladder ran down
into the interior of the submersible.

"What do you say--shall we go below?" questioned Jack.

"I'm willing if you are," answered his cousin.

"And so am I," added Gif. "I don't believe there is any one around."

"Well, we'll take a chance," answered the young captain. "If there is
any alarm, we'll have to run for it."

"Yes, and we may have to fight for it," added Fred.

Jack went down the lad