inutes I was sleep. I awoke
to feel a curious unease, a sense of impending catastrophe. Ray was
bending over me, his face drawn with anxiety.

"Something's happened!" he whispered. "She's gone!"

I sat up, staring into the liquid blue vastness of the tall cylinder
above us.

"Listen! What's that?"

A deep bell-note sounded out, brazen, clanging. Sonorous, throbbing,
mighty, it rang through the cylindered rooms. Slowly it died; faded to
silence with a last ringing pulse. Tense minutes of silence passed.
Again it boomed out, throbbed, and died. After more long minutes there
was yet a third.

"Outside, somewhere!"

Ray started; ran to the arched door. We looked out upon the dense
forest of gold and crimson mushrooms that grew below the black cavern
roof. Before us, across a few hundred yards of bare rocky beach, was
the edge of the crystal lake with the city of blue cylinders upon its
floor.

"God! What's that?" Ray gripped my arm crushingly.

A thin wailing scream came across the beach from the black lake. A
piteous sound it was, plaintive, pleading. Higher and higher it rose,
until it was a piercing silver note. Clear and sweet it was, but
inexpressibly lonely, sorrowful, mournful. It sank slowly, died away.
Again it rose and fell, and again.

"It's Mildred!" I gasped. "Didn't she say something about singing to
the crabs?"

"Yes! I think she did. Well, if that's singing, it's wonderful! Had me
feeling like I'd never see another human. But listen--"

* * * * *

Liquid, trilling notes were rising, pealing out in a queer, swift
rhythm. It was happy, joyous, carefree. The rippling golden tones made
me think of the caroling of birds on a spring morning. Swiftly it rose
and fell, pure and clear as the tinkle of a mountain brook.

Mildred sang not words but notes of pure music.

The gay song died.

And the strong clear voice rose again with the force and challenge of
bugle notes, with a swift marching time beating through it. It
throbbed to a rhythm strange to me. It set my feet tingling to move;
it set my heart to pulsing faster. It was a challenge to action, to
battle.

Unconsciously obeying the suggestion of the song, Ray whispered,
"Let's get over and see what's going on."

We leaped through the door and ran across four hundred yards of rocky
beach to the edge of the lake. We stepped on a granite bluff a few
yards above the water, to gaze upon as strange a sight as men