radiophone:

"They've got me inside, Keith. It's dark, and filled with water. I
can't see anything, but I--I guess we're going through the cavern....
Forget about me, old boy. So long! So--"

The voice was abruptly cut off.

Keith ripped the instrument from his head. Then, face white and drawn,
he ran to the radio cubby. Standing over Sparks' inert body, he put
through a call to Robert Knapp, on the _Falcon_.

"Knapp?" he said harshly. "This is Wells. I'll be with you in a few
minutes. Yes--yes--I'll tell you the whole story later. But get this
now: Have the day shift all ready to take over the submarine by the
time I pull alongside."

He said no more just then; but rang off, and, looking back, he
muttered savagely:

"But I'll be back, Hemmy--I'll be back!"

CHAPTER IV

_In the Cavern_

"That's the story, Knapp. They got Bowman, and I had to run away.
Their ship disappeared into the cavern. I've got a hunch, though, that
it's not just a cavern, but a tunnel, leading through to some
underwater world. That series of sub-sea earthquakes probably opened
it up; and now these devil-octopi are free to pour out. I've _got_ to
find out what's what, and that's why I'm going down again as soon as
the torpedo system's ready!"

Keith and Robert Knapp were in the _Falcon's_ chart room. On the table
before them lay a broad white map with a cross-mark indicating the
position of the mysterious dark cavern.

Wells was striding up and down like a caged tiger in his impatience to
be off. Every other minute he glared down to where the _NX-1_ lay
alongside. On her conning tower stood the tall blond-haired figure of
Graham, the first officer of the day shift, supervising the final
details of the work of installing a system of jury controls whereby
the submarine's torpedoes could be fired from her control room.

Keith stopped short and faced Knapp. "It won't be so one-sided this
time, Bob," he promised. "You see: when the location chart shows the
enemy ship, I'll rush all men into the control room, where the
paralyzing ray can't harm them. I don't know but what they have in
other weapons, but I'm gambling on getting my torps in first. They've
killed Bowman; they've ravaged a whole fishing fleet; they're free to
emerge from their hole and maraud every ocean on the globe! They've
got to be stopped! And since I'm armed and have the only submarine on
the spot, I've got to do it! I know how to fight them now!"

*