He turned. "When we were first returning from the surface it occurred to me as we came out that there had to be some sort of volcanic current warming this cavern, sun or no sun. Now it seems to me that if we could find that current, we would find some sort of air stream or water stream, that must go upward. We ought to look for the warmest spot in the cavern, trace it." Peter turned, a sharp light in his eye. "And now that you say it, do you know what that current is? It's the one that passes the break in the crater wall--the constant hurricane that we broke through to get in here, which rushes by the break so hard and so fast that it seals this cavern's quiet inner air as perfectly as if it were an air lock. It has to be that very current which passes somewhere lower down and warms this cleft!" Robin nodded, a sharp excitement stirring him. "I think we have hit on it. The night time is the time to hunt for it. Find the spot or places in this cleft that stay warmest and they must be nearest the underground wind tunnel." So they set out on a new course of exploration, this time scouting the bubble in the dark of the night. It grew chillier, but in their space suits, which they had resumed for this expedition, they could keep warm. They found several areas along the ground where it seemed a bit warmer than in the cave generally, but after several days of search, this clue also seemed fruitless. The areas were such that no amount of digging short of high explosives would suffice. Finally when the long Lunar night was almost over, they awakened from sleep in the cave to face the thought that this too was a blind alley. Korree entered, the monkey on his shoulder. He made his way to them, noticed their air of sadness, asked them why. Tired, Robin explained to him what they were looking for. His Glassie friend cocked his head. "You come my home. I show you hot spot," he said. The two men looked up. "What?" asked Peter. Korree repeated his statement. Peter looked at Robin quizzically. Without another word the two got up and followed the Glassie. The deep cave where the chief made his home was only a short distance from their own. Here, at the very back of the chief's home, they found what they sought. There was a thin, sharp crack in the rear wall. The stone around it was definitely warmer than that in the rest of the cave. Putting their ears to the crack, they could hear the faint high whistling of the air current that must be roaring past only a foot or two beyond. "This is why it was picked as the chief's cave," said Robin. "It's practically air-conditioned!" The next day, after the sun had finally made its appearance, the two started to work in the back of Korree's cave. They worked carefully with axe and pick, enlarging the crack, chipping away at it. Finally, they dislodged a sizable segment of rock, enough to allow one man to squeeze through.