The quartet gathered together near the door and watched with amazement as six more of the huge diggers appeared, grunting and rooting at the soil like mammoth pigs. Two crashed together with a terrific clank of metal and their wheels dug up the ground as they hung locked together, whirring thunderously. The four went into the tower, down the metal corridors to the elevator which took them to the floor with the control boards. Beta pressed a combination on the buttons of the door which shut off the room with the central boards. The door whined slowly open. "The door's warped," Alpha observed. "There must have been an explosion inside." They entered the room and at first nothing seemed wrong. The great metal vault in the center of the room seemed intact, the walls were whole. Beta walked around the vault, which rose like a fat column from the metal floor to the ceiling. Alpha came to his side when he heard him exclaim. "Look at the wall," Beta said. * * * * * There was a hole the size of a basketball in one side of the wall. The metal about it was melted, bulging out in a fringe around the hole and running down the wall to the floor. Alpha traced the probable trajectory of the missile which had made the hole and found another hole, much smaller, on the face of the vault. The walls of the vault were buckled slightly inward. The two Tindars stared from one hold to the other in amazement. The Kantons came to their side. "No meteor did that!" Miss Kanton's dupe said. "It's hardly possible, unless ..." Beta paused. "Power was packed into it, whatever it was!" "It penetrated three feet of tension steel at the very least," Alpha said. "Nothing as small as this meteor seems to me could have done that." "Unless ..." Beta paused again. "The momentum is what counted. Suppose that the mass was terrific, the speed equally terrific?" Alpha and Beta snapped their fingers at the same instant. "Neutronium!" they said. "We've found several bits of it already in space," Alpha added. "Terrific mass. That's the only possible solution." "A piece of neutronium, of high velocity, accelerated by the gravity of Bolus, plus opposite velocities. That would have done it," Miss Kanton said. They were speaking quickly, their keen minds suddenly tearing the veil from the problem with a scientific hunger. "That's that, then," Alpha sighed as if it had been too easy. "Wrong," Miss Kanton's dupe interposed. "Look at these meters!" The other three went to her side. The meters were jumping crazily from maximum to minimum, their needles bent and twisted. Another type was rapidly clicking off numerals on its way down to zero. Miss Kanton tapped it. "That's the fuel tanks," she said. "They're draining somewhere!" Alpha said. "Down, naturally!" the girl-dupe said. "The engine room covers the whole underground floor! If--"