Sounding is a historical nautical term for measuring depth. The term probably originated from the expression of sounding the well — the well being a shaftlike structure in old sailing vessels that reached all the way to the lowest part of the bilge. This shaft gave cargo free (uncluttered) access by the ships carpenter to the bilge spaces so that he might determine whether the ship was taking on water. That is, was the hull "sound", as in healthy.
The term was also used for measuring depth when navigating in confined waters, such as a harbor entry or on the occasion of making landfall. Soundings from small boats were usually taken using a sounding pole, but deep soundings above 6 fathoms (11 m) were impractible so the practice then was to sound the depth using a line and lead (pronounced "leed"). The was a four pound (2 kg) lead-filled iron or iron bound weight, usually cylinder shaped with a concave bottom, and always with a hookeye, to which the sounding line was attached. The sounding line was knotted at various intervals and had odd objects of a distinctive nature at each major interval (fathom=6 ft) that could be readily touched and identified even in pitch darkness. Sometimes soundings were taken to establish position, a navigation function then, rather than safety alone. Soundings of this type were usually taken using tallow coated leads with a big wad of tallow (a soft waxlike semi-sticky substance) in the bottom concavity. The tallow would bring up part of the bottom sediment (sand, pebbles, clay, shells, etc.) and allow the ship's officers to better estimate their position.
Traditional terms for soundings are a source for several important common expressions in the English language, notably "deep six" (a sounding of 6 fathoms) and Mark Twain (from "by the mark, twain", for a 2 fathoms). The term lives on in today's world in echo sounding, the technique of using sonar to measure depth. See Fishfinder (fathometer).