Describe symbolism in The Basic Training of Pavlo Hummel by David Rabe

Sgt. Brisbey's anecdote about the explorer Magellan symbolizes a central theme of the play: Magellan, according to Brisbey, wanted to know the depth of the ocean on which he was sailing, so he dropped a rope of two hundred feet over the side of his ship. "He thinks because all the rope he's got can't touch bottom, he's over the deepest part of the ocean. He doesn't know the real question. How far beyond all the rope you got is the bottom*?"