Tom Swift and His Big Tunnel

Tom Swift, seated in his laboratory engaged in trying to solve a puzzling question that had arisen over one of his inventions, was startled by a loud knock on the door. So emphatic, in fact, was the summons that the door trembled, and Tom started to his feet in some alarm. "Hello there!" he cried. "Don't break the door, Koku!" and then he laughed. "No one but my giant would knock like that," he said to himself. "He never does seem able to do things gently. But I wonder why he is knocking. I...