 Why did Pop go to Lunar City?
The shack and the job he filled were located in the medieval notion of the physical appearance of hell. Once in two weeks Earth-time a rocketship came around the horizon from Lunar City with stores for the colony deep underground. Come nightfall Pop lowered the supplies down the long cable into the Big Crack to the colony far down inside, and freshened up the landing field marks with magnesium marking-powder if a rocket-blast had blurred them. 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. There is nothing like it on Earth, of course. 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. Even now there were some who had to be carried, but there were some tougher ones who were able to walk to the rocketship if Pop put a tarpaulin over their heads so they didn't have to see the sky.
 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.