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." Peter shone the lamp across and down the cave. The passage cleared a long, tunnellike channel which ran down into darkness on one side and away into equal darkness on the other. Only a few yards across from them he could see the gray surface of the wall. There seemed to be nothing else except the whistling noise. "Edge along the wall here carefully," said Peter, and started off. He kept one shoulder rubbing the wall near them and walked carefully down the passage. Robin edged out, following him closely. He felt no movement of air, yet he detected a faint trace of warmth on his outer side. Somewhere, invisible to him, that cataract of volcanic air was flowing. Was it a few feet or a fraction of an inch? He could not tell. The wall bellied wider a little, allowing a chance to get farther away from the unseen wind. Peter was waiting here. "I think we'd better adjust our space equipment now. We have a short way to go, then we'll have to fight our way across that air blast. There's an opening to the surface at one point nearby. Once we cross the wind and get to it, we'll be outside." Robin let down his pack. Peter examined Robin's equipment again, looking worried. He shook his head once or twice. "I hope it works out all right, but some changes will have to be made."