"You know, it's good to realize that my dream is about to become a reality. If it hadn't been for Phil.... Or no--I really ought to thank you, Sue. You're the one responsible for his participation!" And he smiled fondly at the slender girl by his side. "Phil joined us just for the scientific interest, and for the thrill of going four miles down into the earth," she retorted at once, in spite of the blush her father saw on her face. But he did not insist. Once more he turned, as to a magnet, to the machine that was his handiwork. The fifteen-foot sphere was an earth-borer--Guinness's own invention. In it he had utilized for the first time for boring purposes the newly developed atomic disintegrators. Many holes equally spaced over the sphere were the outlets for the dissolving ray--most of them on the bottom and alternating with them on the bottom and sides were the outlets of powerful rocket propulsion tubes, which would enable it to rise easily from the hole it would presently blast into the earth. A small, tight-fitting door gave entrance to the double-walled interior, where, in spite of the space taken up by batteries and mechanisms and an enclosed gyroscope for keeping the borer on an even keel, there was room for several people. The earth-borer had been designed not so much for scientific investigation as the specific purpose of reaching a rich store of radium ore buried four miles below the Guinness desert camp. Many geologists and mining engineers knew that the radium was there, for their instruments had proven it often; but no one up to then knew how to get to it. David Guinness did--first. The borer had been constructed in his laboratory in San Francisco, then dismantled and freighted to the little desert town of Palmdale, from whence Holmes had brought the parts to their isolated camp by truck. Strict secrecy had been kept. Rather than risk assistants they had done all the work themselves. * * * * * Fifteen minutes passed by, while the slight figure of the inventor puttered about the interior of the sphere, brightly lit by a detachable searchlight, inspecting all mechanisms in preparation for their descent. Sue stood by the door watching him, now and then turning to scan the desert for the returning Phil. It was then, startlingly sudden, that there cracked through the velvet night the faint, distant sound of a gun. And it came from the direction of the water-hole. Sue's face went white, and she trembled. Without a word her father stepped out of the borer and looked at her. "That was a gun!" he said. "Phil didn't have one with him, did he?" "No," Sue whispered. "And--why, there's nobody within miles of here!" The two looked at each other with alarm and wonder. Then, from one of the broken patches of scrub that ringed the space in which the borer stood, came a mocking voice. "Ah, you're mistaken, Sue," it affirmed. "But that was a gun."