fathom

n. (pl. Often fathom when prec. By a number) measure of six feet, esp. In depth soundings. v. 1 comprehend. 2 measure the depth of (water).  fathomable adj. [old english]

n. A measure of length, containing six feet; the space to which a man can extend his arms; -- used chiefly in measuring cables, cordage, and the depth of navigable water by soundings.