The term shoal is an example of a word that can have two contradictory meanings ( Antagonym ).
Very shallow depth: A shoal ( with privatory un) is an area in a body of water with very low water depth. This meaning is attested since the 10th century. In shipping, a shoal is an area which poses a risk to shipping because of its lower depths. In addition to the risk of grounding also a dangerously high waves can form up to the underground lakes at depths. Shoals are shown for example on charts and are often marked by floating navaids.
Huge Depth: The word shoal ( with steigerndem un) can also be a great unfathomable depth, respectively. This meaning is occupied only since the late 18th century and first Upper German. The prefix un - stressing and reinforcing effect here, similar to the words gazillion or enormous sum ( Augmentativbildung ).
The lighthouse Pulaski Shoal Light marked a shoal in the northeast of the Dry Tortugas
Marking a shoal with an artificial island ( Schalch in Chiemsee )
Rocky shoals in the Upper Rhine protrude above the water level ( Krausaue )