His career as a riverboat pilot was marred by tragedy

Since he was a small lad, Twain had always wished to become a riverboat captain. At the time, it was one of the most coveted positions. After two years of training, he received his license. Clemens started working as a trainee riverboat pilot on the Mississippi River in 1857. He secured his younger brother Henry a position on board Pennsylvania the next year while working there. Up until early June, Samuel Clemens worked in Pennsylvania. Then, on June 13, tragedy struck when a boiler explosion aboard Pennsylvania, which was headed toward Memphis, killed many people, including 19-year-old Henry. Despite being horrified by the experience, Samuel Clemens obtained his pilot's license in 1859. Up to the start of the American Civil War in 1861, when commercial activity down the Mississippi was prohibited, he worked aboard steamboats. From that, he got the pseudonym Mark Twain from a steamboat.

Mark Twain (real name Samuel Langhorne Clemens) decided to choose a pseudonym early in his writing career, just like many of history's greatest authors. Before settling on Mark Twain, which is nautical slang for two fathoms (12 feet) deep, he tested out many other names, including Thomas Jefferson Snodgrass, Sergeant Fathom, and Josh. When referring to two, a pilot will use "twain." A leadsman would signal that the river depth was safe for the steamboat by calling "by the mark twain."