The depth gauge showed two hundred feet. Already the three people were numb from the vibration; they hardly felt any sensation at all, save one of great weight pressing inwards. The compartment was fairly cool and the air good--kept so by the automatic air rectifiers and the insulation, which shut out the heat born of their passage. Quade had been carefully watching Guinness's manipulation of the controls, when he was struck by a thought. At once he stood up, and shouted in the elderly inventor's ear: "Try the rockets! I want to be sure this thing will go back up!" Without a word Guinness shoved back the lever controlling the disintegrators, at the same time whirling a small wheel full over. The thudding drone died away to a whisper, and was replaced by sharper thundering, as the stream of the propulsion rockets beneath the sphere was released. A delicate needle trembled on a gauge, danced at the figure two hundred, then crept back to one-ninety ... one-sixty ... one-forty.... Quade's eyes took in everything. "Excellent, Guinness!" he yelled. "Now--down once more!" The rockets were slowly cut; the borer jarred at the bottom of its hole; again the disintegrators droned out. The sphere dug rapidly into the warm ground, biting lower and lower. At ten miles an hour it blasted a path to depths hitherto unattainable to man, sweeping away rock and gravel and sand--everything that stood in its way. The depth gauge rose to two thousand, then steadily to three and four. So it went on for nearly half an hour. At the end of that time, at a depth of nearly four miles, Quade got stiffly to his feet and once more shouted into the professor's ear. "We ought to be close to that radium, now," he said. "I think--" But his words stopped short. The floor of the sphere suddenly fell away from their feet, and they felt themselves tumbled into a wild plunge. The drone of the disintegrators, hitherto muffled by the earth they bit into, rose to a hollow scream. Before the professor quite knew what was happening, there was a stunning crash, a shriek of tortured metal--and the earth-borer rocked and lay still.... * * * * * The whole world seemed to be filled with thunder when David Guinness came back to consciousness. He opened his eyes and stared up into a darkness to which it took him some time to accustom himself. When he did, he made out hazily that he was lying on the floor of a vast dark cavern. He could dimly see its jagged roof, perhaps fifty feet above. There was the strong smell of damp earth in his nostrils; his head was splitting from the steady drone in his ear-drums. Suddenly he remembered what had happened. He groaned slightly and tried to sit up. But he could not. His arms and legs were tied. Someone had removed him from the earth-borer and bound him on the floor of the cavern they had plunged into.