The term mark twain is a navigational description of two fathoms, meaning a depth of twelve feet of water, derived from the measurement of a mark combined with "twain" meaning "two." Mark twain is considered the lowest depth for safe water travel. The measurement of a mark is taken with a "hand lead" that consists of a rope with a heavy weight fastened to its end. The rope is usually twenty-five fathoms long and is marked in increments of two, three, five, seven, ten, fifteen, seventeen and twenty fathoms.
The process of measuring the water is called "sounding," which was overseen by a leadsman who measures and "sounds out" the depth to the other crew members responsible for sailing or operating the boat in open water. The leadsman would usually stand on a platform, called "the chains" which projected from the ship over the water and "sound" from there. A typical sound would be expressed as "By the mark 7," or whatever the depth was. In modern English language, it is interesting to note that the expression "deep six," refers to this old method of measuring water. In Life on the Mississippi, Twain describes sounding: "Often there is a deal of fun and excitement about sounding, especially if it is a glorious summer day, or a blustering night. But in winter the cold and the peril take most of the fun out of it."
Today, the mark is measured by sonar equipment, but in Mark Twain's day, it was the common method of ensuring that a boat could safely navigate a body of water without becoming stranded on a sandbar, which were plentiful in the Mississippi River. As a steamboat pilot, Twain would be especially subjected to these marks as he moved his boats up and down that river.
Mark Twain is also significant in the literary world as it is the pen name of Samuel Clemens, the author. Legend has it that Twain actually "lifted" the pseudonym from his supervising captain, who often contributed articles to local papers. Regardless of how Mark Twain, the writer, devised the name, it has become synonymous with tales of adventure, mischief, mishap and life along the Mississippi River, namely in the fictional town of St. Petersburg, known as Hannibal, Missouri in real life. Through many characters, Mark Twain's mark never halted at the twain, it would always be bottomless.