Samuel Langhorne Clemens was known the pen name Mark Twain, a nom de plume he coined to reflect his time as a river boat pilot.  On the Mississippi in the 1800's it was common to measure river depth in terms of 6 foot increments demarcated on a rope by knots.  Thus, when the river was of depth 12 feet, or two fathoms, the boatmen would call out, "Mark Twain" indicating the second of two knots was submerged.