Members of the garrison of this fortress, thousands of years gone by, had visited Earth. One of them, doubtless, had carried that other cube. Why? When the garrison abandoned the asteroid they left these cubes behind. They left behind intricate machinery to call them back. They left squat machines and ten-foot globes which must be weapons. They left nothing that would be useful in the place to which they had removed. But they'd left these cubes, hundreds of thousands of them. The cube, then, could be anything. It could be impersonal, like equipment for the fortress that would be useless elsewhere. The fortress' equipment was designed to deal out death. Were the cubes? No. Burke had owned one without damage. When that cube split into glistening, tissue-thin plates, no one was injured. To be sure, there was his dream. But the cube wasn't a weapon. Whatever else it might be, it was not dangerous. He went into the spaceship and for no reason whatever firmly locked both air-lock doors. Holmes and Keller were asleep. There was no sound from the lower compartment occupied by Sandy and Pam. Burke put the black object on the control-desk. The single cube on Earth had been meaningless. The museum which joyfully accepted Cro-Magnon artifacts from his uncle had dismissed it as of no importance. It was fit only to be given to an eleven-year-old boy. But a roomful of such cubes couldn't be without meaning! He dismissed this newest mystery with an almost violent effort of his will. It was a mystery. Yet there was no intention to have the fortress seem a mystery to whoever answered its call to space. He could guess that the signals were notification of some emergency which needed to be met. The automatic apparatus of the ship-lock was set to aid those who came in response to the call. But everything presupposed that those who came would know why they came. Burke didn't. The thing must be simple, an explanation not yet thought of. But there was nowhere to start to think about it! His recurrent dream? No. That was as mysterious as the rest. Burke was very, very lonely and depressed. He could look for no help in solving the mystery. Earth was now past the point of conjunction with M-387, and moved nearly a million miles a day along its orbit, with nearly half of them away from the fortress. At the most hopeful estimate, it would be three months or later before an emergency space fleet of replicas of his own ship could lift off from Earth for here. And Burke was reasonably sure that the red sparks would have reached the center of the disk in much less time than that. If it were in some fashion like a radar, making a map of the surroundings of the asteroid, the observer's place would be in the middle. In that event, whatever the red sparks represented would reach the fortress before more ships came out from Earth.