What does Sams letter mean in Jack Finnys The Third Level?

Sam’s letter has several important functions. First, as previously noted, it serves to introduce the “third level” of the Grand Central station. Second, it acts as an inciting incident which sets the story into motion. Third, it contains a number of important clues and suggestions about the mysteries that surround this underground world. And fourth, it serves to characterize Sam, presenting him as a mysterious, even somewhat sinister figure.