fathom

A fathom is a unit of length in the imperial and U.S. customary systems equal to 6 feet (1.8288??m), used especially for measuring the depth of water. The fathom is neither an international standard (SI) unit, nor an internationally accepted non-SI unit. Historically, it was the maritime measure of depth in the English-speaking world but, apart from within the US, charts now use metres.