And that's when I learned the primary function of that red spike at the tip of the tail. No sooner were we away from the fungus-lighted chamber, than that tiny trylon began to glow, first pale pink, then a brighter scarlet, and finally a brilliant yellow-orange. We followed that bobbing tailtip like the _ignis fatuus_ through the bowels of Hell. Snow's grip on my hand grew a little tighter as we progressed along the slippery red rock of the nearly circular passage. "A regular candy-coated firefly," I joked, to lighten her mood. "What'll they think of next!" She didn't answer. "Bad joke?" I asked. "No ... it's--Did you notice, Jery? We're going _down_." We did seem to be descending, at that. I could imagine Snow's mind conjuring up tons of planet pressing down on us without warning. "Not down," I said to her. "Downer. If it sets your mind at rest, we just took off from a place way below ground. If the roof didn't fall in there, it probably won't up ahead." "How do you know that?" she asked, her curiosity taking the place of her trepidation, which was what I'd hoped for. "The air," I said. "We were breathing in that chamber, remember? For the air to be that plentiful, we just had to be far under the ground, already. The atmosphere grows denser as one descends, you know; like on the canal bottoms." "I've never been on a canal bottom," she said. "Come to think of it, neither have I! I must have read that someplace." We followed Clatclit and his magic taillight a few more yards, then Snow said, "You don't have to kid around to buck me up, Jery." "Oh, yes I do," I disagreed sincerely. "For some reason or other, my main worry at the moment is for you. So if I can keep you happy, I'm happy. See?" "Uh-huh," she said softly. Her hand pressed mine more tightly for a brief moment. "Thanks." "If you think you can repay my efforts with a mere word of gratitude," I said in a villainous whisper, "you have lots to learn about men, poor child." "Jery, don't joke any more. I'm frightened, really frightened," she said, her voice trembling. "Okay," I said, and left off. I didn't tell her, but my own pulsebeat wouldn't have qualified me for a hero medal, either. Then, up ahead in the blackness beyond Clatclit's glowing tail spike, I heard a dull roaring. A few hundred yards further on, the roar was louder, and I could feel it through the soles of my boots. "What is it, Jery?" Snow whispered. "It sounds like water!" I said. "Like more water than I thought there was on this whole spaceborne Death Valley!" "Jery!" Snow's fingers dug into my palm. "If this is the way to the Ancients, then this must be what Clatclit meant when he told you he could only take you so far and no further!" "Sure it is!" I exclaimed excitedly. "A child could have figured it out. What else but water could impede these rock-hard things!"