One Against the Moon by Donald A. Wollheim

could reach it was a dark tunnel. "Maybe this is what we're looking for," Robin said to Cheeky. "It seems to slant slightly upward. Maybe it will take us to the next bubble."
    Cheeky peered in, walked in slowly and out of sight. "Hey," called Robin, "wait for me!" He followed the scampering monkey.
    Now his lantern proved handy. The glow it shed could barely be noticed in the light of the great bubble, but here in the darkness of the cleft, the pale glow was distinct and definitely illuminated the ground a few feet in front of him. On he walked, holding the lantern ahead of him, watching Cheeky's long tail flick in and out of its circle of dim light, as the monkey would dash ahead and dash back.
    Soon Robin found himself walking in almost total darkness, save for the limited glow of his lantern. The floor of the cleft occasionally slanted sharply, sometimes breaking steeply downward, sometimes necessitating jumps upward into the darkness. In the Moon's light gravity, Robin was a fantastic jumper, but the darkness made the problem very disconcerting. It was a strange thing to have to leap upward into a black void in hopes that what seemed like a wall in front of you would turn out to have a top and be but a giant step upward. He soared in the darkness, not knowing how near or how far the roof of the tunnel was, feeling strangely disembodied, the monkey clinging to his neck in transit.
    He missed several such jumps, managed to avoid being bruised severely only by the feathery softness with which he fell afterward. When the going was straight, Cheeky would leap down and go ahead.
    Suddenly he heard a screeching from the monkey. He stopped, flashed his flashlight. The monkey was clutching the edge of a deep break, a pit cut sharply across the floor of the tunnel. Robin quickly reached the spot, scooped up Cheeky. His flashlight revealed the other side of the pit several yards away. Turning its rays downward, he could see no bottom to this crack within the tunnel. He shuddered, thinking what might have happened had he gone into it. Then, gathering Cheeky, Robin leaped.
    He soared lightly across the abyss and landed safely on the other side. He went on, slowly, carefully.
    A spot of light appeared before him. He stared at it and continued moving forward. The light widened, became the end of the tunnel, became the entrance to something new. He hastened on and burst at last into a new cavern-world, the world of the next bubble.
    It looked much like his old one, but it was definitely smaller. The rounded ceiling could be made out quite clearly and he estimated its diameter as not more than a half mile. The far side of the bubble could be seen clearly and this one, he noticed, even from where he stood, had many such clefts and cracks in the wall. It was, he supposed, either an older bubble, more cracked in the course of eons of moonquakes and heat changes, or else it was more tightly knitted in a close mass of such bubbles.
    A thick jungle of giant Moontrees was growing here, stalklike plants resembling those he knew, but seemingly larger and more profuse. Robin started to walk through it toward the farther wall of the bubble. Cheeky had leaped into the stalks and was swinging through them ahead of him, when suddenly the monkey uttered a terrified shriek and there arose a strange high-pitched barking sound. Robin pushed through and saw the monkey, high in the top of a Moontree and a strange sort of Moonhound leaping for him. This kind of Moonhound was considerably bigger than the ones he had seen in his original bubble. It was uttering the eager bark of a hungry beast sighting its prey.
    Robin unlimbered his bow and fitting an arrow into it, let fly. The sharp missile skewered the Moonbeast and the animal twisted in mid-air and fell thrashing to the ground. Robin dashed in and finished it off with a blow from the stone club he carried.
    Cheeky came down from the tree cautiously, advanced to the dead animal, and prodded it. Then the