We lay down on the silken beds. In a few minutes 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?"