What are the main conflicts and the theme/goal of Herman Melville's Moby Dick?

As a classic, Herman Melville's "Moby Dick" can be read on different levels.  On one level, it is a narrative about whaling and whales and one obsessed sea captain who seeks revenge again a levithian called Moby Dick who cost him his leg in a previous encounter.  This narrative is set in Nantucket, Massachusetts, a site renowned for its craggy seamen and fearless whalers, and is narrated by a young man who signed on the the enigmatic Ahab's ship in order to be gone "for a year or two."

But, since the narrative of the chase for Moby Dick does not occupy more than a fourth of the novel, Melville's work entails much more than the story of a maniac sea captain.  It is, rather, a metaphysical examination of the essence of nature.  For, as Captain Ahab declares early in the novel, what man sees of nature is but an inscrutable "pasteboard mask."  And, Ahab tells his crew that he would break through this mask in order to understand the meaning of existence behind it.  As what is termed a Dark Romantic, Melville and others like him perceived nature as a force that often works against man.  Moby Dick is the "pasteboard mask" that conceals the malevolent force against man, particularly Ahab who desires a rencounter with the "levithian" so that he may "break through that mask."  For, Ahab believes that if he can capture the great white whale, he can conquer the malevolent force against him.

Having remarked that the sea was his school, Melville's novel encompasses several themes, but the overriding one is man's search for meaning in his existence against the dark forces of nature, with the voyage of the Pequod being a metaphor for life.  In Chapter 58, Melville writes,

As the appalling surrounds the verdant land, so in the soul of man there lies one Tahiti, full of peace and joy, but encompassed by all the horrors of the half-known life.  God keep thee!  Push not off from that isle, for thou canst never return.

Once someone has encountered evil--has left the Eden-like isle of innocence and has learned of life--he can never go back to his naivete, but must seek answers as Captain Ahab has felt compelled to do, risking his very life. himself.