(oceanography) A layer of the ocean lying usually 300-500 metres deep composed of dense concentrations of pelagic animal life, that moves depending on the time of day, up at night and down during the day. It sometimes appear as if it were the seabed on sonar scopes, due to the density of life in the layer.

What matters is a person's character, rather than his/her appearance.

Alternative form of still waters run deep

A person with a calm appearance has, or may have, considerable inner emotion, character, or intellect.

Synonym of knee-deep in the Big Muddy

(chiefly US, idiomatic) Stuck in a predicament; mired in a difficult situation, especially one resulting from poor judgment or bad leadership.

Covering a large set of topics but on a very shallow level.

(idiomatic) Between undesirable alternatives.

(transitive) To make deep or deeper / (transitive) To make darker or more intense; to darken / (transitive) To make more poignant or affecting; to increase in degree / (transitive) To make lower in tone

comparative form of deep: more deep