What was the Meaning of ‘Mark Twain’?

Samuel Langhorne Clemens grew up by the Mississippi River, where he was to set his most famous stories- The Adventure of Tam Sawyer and The Adventures of Huckleberry Finn. For several years he worked as a river pilot, guiding traffic through the treacherous waters. Later, as a journalist and author, he chose to write under a pen name taken from one of the calls used by the pilots as they teasted the depth of the water. ‘Mark Twin’ was the term for water that was just safe for navigation.