A fathom is a unit of length in the US customary and imperial system of units that is equal to 6 feet, 2 yards, 72 inches, or 1.8288 meters. A fathom is about the height of a man. Fathom is originated from the British imperial system, and is usually used for measuring the depth of water in the US. In most other parts of the world, meters (m) are used to measure water depth instead.

The fathom is mainly used in nautical contexts, but up until the early 20th century, was also used in the United Kingdom to measure the depth of mines.

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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.