there lay the _Nereid_, apparently unharmed.

But her lights were out and there came no answer to their repeated
calls, so they judged she must be empty.

What had happened to Captain Petersen and his crew? What strange
sub-sea enemy had overcome them? What was now their fate?

Unanswerable question! But one thing was certain. Larry had misjudged
the captain in suspecting him of mutiny. He was sorry for this and
resolved he would make amends by doing all in his power to rescue him
and his men, if they were still living.

Meanwhile his own plight, and that of Diane and her father, was
critical. What was to be done?

Suddenly, as all three stood there debating that question, Professor
Stevens uttered an exclamation and strode toward the pyramid.
Following him with their eyes, they saw him pass through an aperture
where a huge block of stone had been displaced--and disappear within.

The next moment they had joined him, to find themselves in a small
flooded chamber at whose far end a narrow gallery sloped upward at a
sharp angle.

The floor and walls were tiled, they noted, and showed none of the
corrosion of the exterior surfaces. Indeed, so immaculate was the room
that it might have been occupied but yesterday.

As they stood gazing around in wonder, scarcely daring to draw the
natural inferences of this phenomena, there came a rasping sound, and,
turning toward the entrance, they saw a massive section of masonry
descend snugly into place.

They were trapped!

* * * * *

Standing there tense, speechless, they waited, wondering what would be
the next move of this strange enemy who held them now so surely in his
power.

Nor had they long to wait.

Almost immediately, there issued a gurgling sound from the inclined
gallery, and turning their eyes in the direction of this new
phenomena, they saw that the water level was receding, as though under
pressure from above.

"Singular!" muttered Professor Stevens. "A sort of primitive lock. It
seems incredible that human creatures could exist down here, but such
appears to be the case."

Larry had no desire to dispute the assumption, nor had Diane. They
stood there as people might in the imminence of the supernatural,
awaiting they knew not what.

Swiftly the water receded.

Now it was scarcely up to their waists, now plashing about their
ankles, and now the room was empty.

The next moment, there sounded a rush of feet--and do