The nights grew crisp and there was a hint of frost. It was then that they turned all their attention to the house, plastering it all over with mud save at the very top, where the air-holes were. So thick did they lay it on that only here and there did the end of a stick project. Then came a night which made a thin sheet of ice over the pond and froze the mud-plaster of the house. The cold increased. The ice grew thicker and the walls of the house so hard that not even the powerful claws of a bear could tear them open. It was for this that that last coating of mud had been put on. The nights of labor were over at last. There was nothing to do now but sleep on the soft beds of grass or of thin splinters of wood, for some had preferred to make beds of this latter material. For exercise they swam in the quiet waters under the ice. When they were hungry, they slipped down through the water tunnel and out into the pond, swam to the food-pile, got a stick, and took it back to the house, where they gnawed the bark off in comfort and at their ease, afterward carrying the bare stick down to the dam for use in making repairs. Once they discovered that the water was rapidly lowering. This meant a break in the dam. A trapper had cut a hole in it and cunningly placed a trap there. But the wise old leader knew all about traps, and the breach was repaired without harm to any one. Sometimes a lynx or a wolf would come across the ice and prowl around the house, sniffing hungrily as the smell of beaver came out through the tiny air-holes in the roof. But the thick walls were like rock, and Tommy and his companions never even knew of these hungry prowlers. Peace, safety, and contentment reigned under the ice of the beaver-pond. But at last there came a day when a great noise reverberated under the ice. They knew not what it meant and lay shivering with fear. A long time they lay even after it had ceased. Then one of the boldest went for a stick from the food-pile. He did not return. Another went and he did not return. Finally Tommy went, for he was hungry. When he reached the food-pile, he found that it had been fenced in with stout poles driven down into the mud through holes cut in the ice. It was the cutting of these holes that had made the dreadful noise, though Tommy didn’t know it. Around the food-pile he swam until at last he found an opening between the poles of the fence. He hesitated. Then because he was very hungry, he entered. Hardly was he inside when another pole was thrust down through a hole behind him, and he was a prisoner under the ice inside that hateful fence.