As they drew nearer the base of the mountain, the effects of the strange vibrations grew more pronounced. Burl avoided looking directly ahead, keeping his eyes on the ground before his feet, yet even so, he could not help noticing how the stones around them seemed to shimmer in the invisible waves. From the base of the valley the sky now seemed streaked with black and gray rings, as if they were reaching the center of some atmospheric whirlpool. Out of the mountains, after hours of arduous scrambling, they started across the barren rocky plain. Before them rose a vast circular structure several stories high, ominously black and without any sign of windows or doors. Above the building protruded two great projections ending in huge, shining discs. One of the monstrous cuplike discs was facing the Sun, the other pointed in the opposite direction. As the two men came nearer and nearer, the strangeness in the air increased. They felt they were being penetrated through and through with invisible lances, with tiny prickles of heat. "Radiation?" queried Burl softly, afraid of the answer. His father trudged grimly on for a moment, and then put down his pack. He took out a Geiger counter and activated it. He shook his head. "No radioactivity," he said. "Whatever this is, it isn't that." They reached the wall of the building. Oddly, here they seemed sheltered from the unusual vibrations. Burl realized that the source was above them, probably the two mighty discs raised high in the sky. The Dennings surveyed the building, but found no entrance. It must have been a quarter of a mile around its walls, but there was no sign of a door or entry. The wall was of a rocklike substance, but it was not like any rock or plastic Burl had ever seen. "We've got to get in," said Burl as they returned to the starting point, "but how?" His father smiled. "This way." He opened his pack and took two cans of blasting powder from it. "I thought these would come in handy. Lucky we had some left over from the blasting we did last week." He set both cans at the base of the high wall, wired them together, and ran the wire as far as it reached. When the two men were a safe distance away, Mark sparked off the explosive. There was a thunderous roar: rocks and dirt showered around them, and bits of black powdery stuff. When the smoke cleared, Burl and his father leaped to their feet, rifles in hand. There was a crack in the side of the wall where the explosive had gone off. And the rip was large enough to get through! Without a word, they charged across the ground, still smoking from the concussion, and squeezed through the mysterious walls of the enigmatic building. The walls were thin, thin but hard, as befit masters of atomic engineering. Inside, they found a roomless building--one single chamber within the frame of the outer walls.