 Why did Pop go to Lunar City?
Once in two weeks Earth-time a rocketship came around the horizon from Lunar City with stores for the colony deep underground. The Crack, of course, was that gaping rocky fault which stretches nine hundred miles, jaggedly, over the side of the Moon that Earth never sees. Where Pop Young's shack stood it was only a hundred yards, but the colony was a full mile down, in one wall. When it was first found, scientists descended into it to examine the exposed rock-strata and learn the history of the Moon before its craters were made. Pop lived in it all alone, and in his spare time he worked industriously at recovering some missing portions of his life that Sattell had managed to take away from him. There were air-tight bulkheads for safety, and a hydroponic garden to keep the air fresh, and all sorts of things to make life possible for men under if not on the Moon. In the Moon's slight gravity, a man is really adjusted to existence when he has a well-developed case of agoraphobia. He'd shipped out, whimpering, to the Moon to get far away from Pop, and Pop was just about a mile overhead and there was no way to get around him. Pop had come back to consciousness in a hospital with a great wound in his head and no memory of anything that had happened before that moment. All of which happened back on Earth and a long time ago. There was a warning-bell in the shack, and when a rocketship from Lunar City got above the horizon and could send a tight beam, the gong clanged loudly, and Pop got into a vacuum-suit and went out the air lock. He usually reached the moondozer about the time the ship began to brake for landing, and he watched it come in. Instantly the rockets cut off, drums of fuel and air and food came out of the cargo-hatch and Pop swept forward with the dozer. At first Pop followed absorbedly from city to city, to recover the years that had been wiped out by an axe-blow. When Sattell fled to another continent, Pop followed because he had some distinct memories of his wife—and the way he'd felt about her—and some fugitive mental images of his children. When Sattell frenziedly tried to deny knowledge of the murder in Tangier, Pop had come to remember both his children and some of the happiness of his married life. Even when Sattell—whimpering—signed up for Lunar City, Pop tracked him.
 Pop went to Lunar City because Sattell went to Lunar City.
 Pop went to Lunar City because the Earth is overcrowded.  Pop went to Lunar City because Sattell went to Lunar City.  Pop went to Lunar City because his family was murdered, and he couldn't stand to be on Earth any longer.  Pop went to Lunar City to mine diamonds.