A fathom is a unit of length equal to 6 feet. It is used for measuring the depth of water.
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Samuel Langhorne Clemens was one of the most famous Americans of the 1800s. The author of several books, including The Adventures of Tom Sawyer, he is best remembered by his pen name, Mark Twain. This name may have come from his work as a steamboat pilot. To steer the steamboat, the water needed to be 2 fathoms deep, or "by the mark, twain" to be in safe water.