The answer is:
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Deep (superl.) — Extending far below the surface; of great perpendicular dimension (measured from the surface downward, and distinguished from high, which is measured upward); far to the bottom; having a certain depth; as, a deep sea.
Deep (superl.) — Extending far back from the front or outer part; of great horizontal dimension (measured backward from the front or nearer part, mouth, etc.); as, a deep cave or recess or wound; a gallery ten seats deep; a company of soldiers six files deep.
Deep (superl.) — Low in situation; lying far below the general surface; as, a deep valley.
Deep (superl.) — Hard to penetrate or comprehend; profound; -- opposed to shallow or superficial; intricate; mysterious; not obvious; obscure; as, a deep subject or plot.
Deep (superl.) — Of penetrating or far-reaching intellect; not superficial; thoroughly skilled; sagacious; cunning.