What is Hemingway's iceberg theory?

Ernest Hemingway's "iceberg" theory is his strategy of fiction writing in which most of the story is hidden, much like an iceberg underneath the ocean. The 7/8ths of an iceberg that are underwater are the strongest part of the iceberg. In the same way, the strongest part of a story is what is hidden from the reader and revealed later.

For this technique to work, the author must know the entire story but only write an eighth of it, as an iceberg only shows an eighth of itself above the water. The iceberg theory works well in mystery stories, as levels of the plot are omitted, and only the essentials are revealed. Most of the "icebergs" in Hemingway's stories concern the awareness of death.