And then Jimmy remembered the metal tube Linda had given him when they first entered this underground chamber. He drew forth the tube and with a quick motion threw it before him. Nothing! The crypt remained steeped in silence. "What was that you threw?" demanded Garth. "Answer, damn you!" Jimmy shrugged. "It was a tube of setro-frenalot--NSK 54," he said. "I think you know what that means, Mr. Garth. The double detonation explosive. If it doesn't explode upon the first impact, the slightest jar, the slightest whisper of sound will discharge it." Garth's face went black with rage. "You damned double-crossing--!" He tossed his heat gun to one of the two pseudo-I.P. men and plunged into the vault. Halfway the significance of Jimmy's words came home to him. Gingerly, a step at a time, he began to work his way toward the metal tube that lay in the light of his electric torch. Now he stood directly above it. He reached down, let his fingers fasten about the tube. With the greatest of care, he lifted it and began to catwalk back to the door of the vault. But at the threshold Jimmy uttered a cry of alarm and swept Linda protectingly into his arms. "What's the matter?" Garth demanded. "The calibo-marset fire. Blue flame. It's started in the setro-frenalot. It's going to go off." Garth's eyes shot wide with fear. He looked down at the tube in his hands, then abruptly swung and hurled it through the open doorway into the vault. There was a low roar, mounting to a crescendo report. A cloud of smoke belched outward, and the ground beneath their feet trembled. At the first indication of Garth's action, Jimmy, Linda, and Hanley had hurled themselves backward, away from the vault door. Garth too had whirled and leaped like a released spring to safety. But the two I.P. men were caught. They had not heard Jimmy's exclamation--hadn't time to guess what was coming. An avalanche of rubble and huge stones washed forward to sweep relentlessly over them. An instant later only a sound of dust-rising debris and masonry fragments marked the spot where they had stood. As the deafening reverberations rolled back into silence, Hamilton Garth seemed to grasp the significance of the situation like a man in a dream. For a moment he stood there, rigid, eyes narrowing, lips quivering. Then with a snarl of profanity, he charged straight at Jimmy Starr.