The sun came up again on the surface and flooded the cleft with its light. The Moontrees grew rapidly in dense profusion. The two brothers gradually explored the length and breadth of the little world, systematically working around it in search of some new path upward.

But their search seemed fruitless. There were a number of holes and breaks in the walls and caves, but none promised a place of exit to the surface.

They went back to the original ledge and tunnel, tried to work their way in, but it was blocked with fallen stone and jammed too tightly for passage.

They discussed the possibility of making explosives, blasting through, but discarded this as they realized the basic fragility of the whole cleft setup. Such blastings might do worse damage, might even crack a direct opening to the surface through which the air within the cleft-bubble would rush out, leaving it a sterile, cold, and dead region.

Finally after another Lunar night and another Lunar day, exhausting still one more Earth month, they settled down to a slow steady picking and shoveling. They worked in the blocked tunnel in all their spare time, carefully picking away chips of rock, pushing others aside, burrowing around fallen slabs, slowly, gradually, painfully working their way along the old path. But it was hard and unrewarding work. It went slowly and they were always afraid of a cave-in.

Two or three times such an event did occur, and had it not been for the slowness with which things fell on the Moon, one or the other brother would surely have been pinned down. On the third such disaster, the two quit the task, returned to their home in the Glassie village discouraged.

"This will not work," said Robin. "We'll have to give up this entire approach. It would be months or even years before we could make our entire way and by that time one of us would surely be killed in the tunnels. They are still highly unsettled, still shifting."

They sat down, looked at each other. "There must still be a way," said Peter. "We must find a way to reach the surface. Otherwise we will remain here forever."

Robin nodded, deep in thought. Another night was coming over the cleft. The sun was passing swiftly from overhead. A chill began to touch the air, as darkness blacked out the cavern. It would be another two weeks before they could resume any work on their problem. Robin started to build a fire in their cave, one they burned every Moon winter's night. As he did so a thought struck him.

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."