What is the worst possible thing for humans to contemplate? That there is no meaning whatsoever to our existence, to our suffering, to our selves, that the universe is random and impersonal.
When Captain Ahab tries to explain to Starbuck why he hates Moby-Dick past all reason and good sense, he goes on and on about looking beyond life's pasteboard masks. Moby-Dick is a hated pasteboard mask disguising the truth. Then Ahab says, "Sometimes I think there's naught beyond. But 'tis enough." He fearfully pulls back from the thought that there is nothing beyond life's surface. It is too unbearable to think that there might be no meaning to the life-shattering loss of his leg.
And in Melville's famously enigmatic chapter on symbolism called "The Whiteness of the Whale," Ishmael asserts that "some certain significance lurks in all things, else all things are little worth, and the round world itself but an empty cipher." The idea that there might be no meaning to life is presented as so ridiculous that it's not worth dwelling on. And to a mid-19th century mind with an absolute belief in the existence of God and His Divine Plan centred on humans, the idea that existence could be random and meaningless would indeed be too crazy to think about even for a second.
But that idea is not crazy to modern existential thought. And I don't think that idea was crazy to Melville. I think he was a hundred years before his time and dared to explore an awful possibility that others could not even imagine. Captain Ahab rages against Moby-Dick to the bitter end but in the last analysis, he is powerless against the awful truth of Moby-Dick.