Fathom

A fathom is 6 feet, 1.8288 m exactly, the traditional unit of water depth and the length of sounding line and chain. Charted depths on older British Admiralty charts were given in fathoms and feet before the metric changeover; most modern charts now use meters, but fathom charts remain in service and the unit survives in lead-line work and cordage. The word derives from the span of a man’s outstretched arms.