Source : Webster's Revised Unabridged Dictionary (1913)

Deep-sea \Deep"-sea`\, a.
   Of or pertaining to the deeper parts of the sea; as, a
   deep-sea line (i. e., a line to take soundings at a great
   depth); deep-sea lead; deep-sea soundings, explorations, etc.

Source : WordNet®

deep-sea
     adj : of or taking place in the deeper parts of the sea; "deep-sea
           fishing"; "deep-sea exploration"