Soon she could dimly discern the dark mass of the opposite side. The fliers were steadily approaching this, and all the time going deeper and deeper. Once Mona turned her eyes searching to the right and left; whereupon Billie was still further mystified to see that, although the cleft was fifty or sixty miles in length, yet its extreme ends seemed entirely open to the world. Nothing but a deep "V" of blue sky was to be seen in either direction. The sun disappeared altogether. Always the two walls grew closer and closer together, until at last Billie could see, despite the semidarkness, a heavy growth of vegetation on the opposite wall. Beneath her, as well, the surface was densely wooded. Still they descended! It was unbelievable; surely the chasm did not extend right into the heart of the globe. They had been flying for hours! At last came a time when the far wall of the cleft was so near that Billie could have shot a deer upon it. She estimated the distance at two hundred yards; and then, and not until then, did she realize that Mona, in order to inspect this bank, was now LOOKING up. The wall which had seemed right ahead, all along, was now actually overhead! Were they entering some sort of a cave? If so, it had dimensions that staggered the imagination. What was more, if it were a cave, how could the mind of man account for vegetation on its roof? Within a few minutes Fort called from his machine; whereupon Mona located a landing-place, a small clearing dimly visible in the distance. The opposite wall of the chasm--or the roof of the cave, whichever it was--now approached to within five yards of the tops of the two machines. Mona and the athlete stepped out, and looked around. Billie's senses swam. This clearing, as has been said, was only a few yards away from the tops of the bushes on the roof. Moreover, all this vegetation, instead of growing at right angles from the surface in the usual way, was all lying flat against the soil, and all pointing in one direction--back, the way they had come! The sky was not overhead any longer; it was a mere strip of very dark blue, lying far off on the horizon. That is, to the right; on the left, the cavelike cleft extended still further, its limits shrouded in darkness. "Queer, isn't it?" laughed Fort. "Shall we walk around?" Whereupon the two young people set out on a narrow, but much worn trail. Keeping the sky always at their right, they passed through the thicket, Mona's eyes telling Billie that the queer horizontal vegetation grew always toward the light. It was much like the growth at the bottom of any gulch; only the two were walking in the normal way, upright, at right angles to the surface, quite as though it were level ground! Overhead the thicket grew in the same fashion; Billie thought the foliage much like ferns. Here and there, however, was a small flowering shrub; and it was to one of these that a tiny, orange-colored bird came flying.