Peter shook his head. "I doubt it. First, we'd probably have been able to hear him coming. Second, he'd still know enough to go get his space suit before following us. Third, he won't remember anything of this trip and will have to find his way."
Rested, Robin gave Peter a boost, hoisting him as high as he could to the top of the cave-bubble. Peter jumped the short distance remaining, catching a grip on the edge of the hole in the cave ceiling. He pulled himself up, then dropped his nylon cord down for Robin to grasp and help himself up.
Up above there was still another small bubble, broken on one side. A whole series of broken bubbles lay revealed, and they walked along this section gingerly. This area was greatly cracked and seamed. It was clear to them that there was a possibility of a fall-in.
Beyond that group they came to another break leading upward, and again they moved on. Now Robin found himself breathing very heavily. "I'm getting very tired," he gasped at last.
Peter stopped. They were still in the break and a severe slope was rising before them. "It's the air pressure. It's getting quite low already. You've been used to the low pressure of the bubbles below, as you tell me, but we are close to the surface and the limited amount of air sealed in this particular bubble-system is thinning beyond the safety point. We'll have to go slow and rest often. I don't want to have to use our oxygen supplies until we are at the limit of our natural abilities."
Robin finally caught his breath, felt power returning. Now the two pushed on, going very carefully and slowly, with rests every few steps.
The steep rise ended at a narrow opening. Peter paused here, motioned to Robin to join him. "This is the crisis point," he said. "Listen."
Robin strained his ears. He was aware of the pounding of his heart struggling for oxygen. He was aware of a ringing in his ears from the low pressure. But now he heard over that a thin whistling, a high, steady rustling whistle coming from somewhere across the narrow, long cave he was looking in upon.
"What is it?" he whispered.
"A most unusual phenomenon," whispered Peter back. "The only thing that keeps the air in all this subterranean region from being sucked away to the surface. It's a volcanic current of hot gas, racing through this long channel at tremendous speed. It must come up from somewhere in the still-warm interior; it must be rushing to some vast cold spot below. But it serves as an effective curtain cutting off the stale air on this side from the near-vacuum of the surface. Its density, velocity, and heat perform the miracle."