Samuel Clemens, later known as Mark Twain, was born in Florida, Missouri, on November 30, 1835. Clemens piloted boats for two years until the Civil War halted steamboat traffic. During his time as a pilot, he picked up the term “Mark Twain,” a boatman’s call noting that the river was only two fathoms deep, the minimum depth for safe navigation. When Clemens returned to writing in 1861, working for the Virginia City Territorial Enterprise, he wrote a humorous travel letter signed by “Mark Twain” and continued to use the pseudonym for nearly 50 years.

Source: History