he muttered to Graham. "You see, I was
right! It's an underground sea--and we're at the top of it." For the
instruments indicated a depth beneath them of roughly three miles.
They were in, evidently, a large cavern, of vast length and depth.

The _NX-1_ continued slowly forward, two pairs of eyes intent on her
teleview screen. Keith jotted down the tunnel's position, and the
funnel-shaped hole sank away behind their slow screws. And then, upon
the location chart, a faint red dot suddenly glowed!

It was upon them in a flash. A small tube of metal, shaped somewhat in
the form of the big octopi submarine, had darted up from below,
hovered a second close to them, and then, almost before they realized
they were being surveyed, sped back into the mysterious depths from
which it had come.

"A lookout, I suppose," Keith muttered, breathing more easily.
"Couldn't have held more than two of those creatures.... Well, the
alarm's out, I guess, Graham, but it can't be helped. Let's see what
it's like down below."

* * * * *

They plunged steadily down, then ahead. And presently there grew on
the teleview vague forms which widened their eyes and made their
breath come quicker. Keith had guessed the tunnel led to a
civilization of some kind, but he was not prepared for the sight that
loomed hazily through the soft blue water.

Strange, moundlike shapes appeared far below, mounds grouped in
orderly rows and clusters, with streets running between them, thronged
with tiny, spidery dots. Octopi! It was, the commander realized, a
city of the monsters--a complete city like those of surface peoples!
For several miles in every direction the water-city spread out,
farther than the teleview could pierce. Wells marveled at this
separately developed civilization, this deep-buried realm of octopi
whose unexpected intellectual powers had permitted such development.
Perhaps, he pondered, this city was only one of many; perhaps only a
village. He could but vaguely glimpse the queer mound buildings, but
saw that they were of varying height and were filled with dark round
entrance holes, through which the creatures streamed on their
different errands....

He saw no schools of fish around. "I guess they're been all killed
off, or eaten," he commented to the wonder-struck Graham. "Probably
the octopi have separate hatcheries where they raise them for food."

"But--good Lord!" the first officer exclaimed. "A city-