An interesting tit-bit on how Mark Twain became Samuel Langhorne Clemens pen name.
Having failed to find a ship that would take him to South America and
the fortune he proposed to make from coca, by the age of 23 he had
become a Mississippi-steamboat pilot. It was a job he held just
briefly, but the memory of the river, its enchantments and dangers,
found its way years later into his most powerful book, Adventures of
Huckleberry Finn. It also found its way into his pen name. Mark Twain,
the name he began to write under in 1863, was a river man's term
meaning a depth of two fathoms, or 12 ft. (3.7 m).