The question of time among the Glassies was an odd one. The Earthling had surmised as much in his observations of Korree. There seemed to be no effort to divide the periods into rest and work. Some hunted and worked when they felt like it, others slept at the same time. When the time came, Robin and Korree made their way out of the cavern upward along a ledge on one side of the bubble wall, through a fault higher up and began to climb a sloping tunnel. For several more days they traveled, always working upward, passing through bubbles of gradually diminishing diameter and sparser vegetation. At one point they waded through a shallow pond, at another they choked in a sulfury cloud of gas that hung about. They squeezed through ever tighter cracks, and the air began to get distinctly thinner and harder to breathe. They were both getting exhausted quite easily; Robin knew they were nearing the surface and the spongy mass of the Moon's interior was tightening. Then at last they stood in a tiny spherical bubble and gazed at a pool of brackish water at one end. There were no cracks in this little cave, no further tunnel or means of progress. "What now?" asked Robin, turning to his companion. Had they taken the wrong turn and come to a dead end? Korree went over to the water pool. He gestured at it, made motions of holding his breath. "We go down in here, move under and come up ... out." He waved a hand in a down-and-under gesture. Robin looked into the water. Maybe the Glassie was right. It was possible that the water at the bottom passed into a fault and led into another cavern. But could he risk it? Korree nodded and without another word, suddenly jumped into the water, spear and all, and vanished. Robin waited. In a little while Korree's head appeared again and the Glassie climbed out. "Tunnel over there," he said, waving beyond the wall of the bubble. "Go up sharp." Well, there was nothing to do but to try it. Robin set down his pack and thought a moment. Cheeky the monkey was scampering around the floor of the small bubble. Robin took off his jacket and shoes, took out of his pocket anything that might be damaged by water, and leaped into the pool. It was an eerie sensation. The water was as dense as on Earth but its weight was so much less. It seemed almost to lack substance as Robin pushed through it, dived deep, and let himself come up again as far as possible. He broke water in total darkness. He was outside the cavern, but exactly where he could not tell. Korree with his light organ had known and that was sufficient. Robin reached for a bank, felt a sloping wall. He grabbed it, pulled himself up in the darkness. That much was right. There was a tunnel here running steeply upward. He sniffed the air. It was strange--breathable, but strange. This part of the Moon enclosure was certainly cut off from the other sections, that was certain.