“What do you suppose made that smooth-walled crater?” Lewis queried of his friend. “Of course the moon is porous and the task was small compared with one on earth of similar proportions, but why do you suppose the walls were so smoothly laid ?”

('Captive of the Crater' by DD Sharp)

“I’ve thought of that,” Pike answered, “and we found something that reveals a good deal. Beyond the shaft on the depressed side is quite a large city still protruding through the fall of meteorites and ash. While waiting for you we found some machinery, ancient to be sure, but still in a fair state of preservation. Of course there will never be any more decay to anything on the moon’s surface, but this machinery evidently dated back to a time when there was water in the depression. There is a much deeper sink beginning less than a mile from the city. It is my idea that when all the water from the great depression was finally drained into the shaft and pumped from the other end, a city sprang up here to use and pump the water from the lowest basin.
“You see, Lewis, my idea is that they dug this shaft from the crater on the earth side of the moon to tap the great lunar sea on the other side, which was probably fresh water. They must have sacrificed thousands of lives, when the final excavation was made, for the water would naturally rush to the far side of the shaft to seek a level with the water in the great depression. At first that was probably high enough to put the water quite near the surface on the other side of the moon. This condition no doubt existed for many thousand years.”
“You’ve got a great imagination. Pike,” Lewis said with a tolerant chuckle, “but how do you account for the smooth walls ? The moon is porous and water falling over the walls could hardly have filled them top to bottom with solid rock.”
“Proving my point,” Pike insisted, “That wall is masonry. The moon creatures probably knew that if they did not wall in the shaft the water would be wasted in the giant caverns of the interior."