Synonyms for Deep:

adj.

dark (adjective)
mystifying.
deep (adjective)
fathomless, inmost, profound, abysmal, bottomless, sunken.
extending very far, usually down (adjective)
inmost, wide, profound, buried, bottomless, fathomless, far, broad, yawning, submarine, subterranean, Immersed, distant, abyssal, subaqueous, beneath, low, below, deep-seated, rooted, abysmal, underground.
intensive (adjective)
in depth.
resonant (adjective)
plangent.
unfathomable (adjective)
abyssal, unplumbed.

adv.

all (adverb)
deeply, late.
completely, intensely (adverb)
acutely.

n.

fathom, bottommost, come down, in difficulty, suffering, knee-deep, having trouble, in trouble at trouble. chesty, pulmonary, breathless, choke, bronchial, heavily, indrawn, surface, labored. alto, contralto. depth, high, gulf, abysm. deep (noun)
deeply, recondite, walk-in, bass, cryptical, mysterious, large, inexplicable, esoteric, in depth, unplumbed, oceanic abyss, wide, unfathomed, profound, broad, cryptic, deep water, abyssal, big, late, abstruse, mystifying, incomprehensible, wakeless, abysmal, trench, sound, inscrutable, low-pitched, colorful, thick, rich, unsounded, intense, bottomless, colourful, distant, low, heavy, artful.
ocean (noun)
main, brine, sea, ocean, brink.
pit (noun)
chasm, well, abyss, crater, cavity, crevasse, pit, bottomless pit, shaft.
the sea (noun)
main, ocean, brine.

Other synonyms:

burning, gulf. casual, depth, breathless. substantial. broken, choke. deep-water
deep water.
intense
intense.
sincere
heartfelt.

Usage examples for deep

  1. Again, what has never in any deep sense been a man's own, cannot surely continue to be his afterwards. – The Seaboard Parish, Complete by George MacDonald
  2. The cut is not deep – After a Shadow, and Other Stories by T. S. Arthur
  3. The water was deep and the life's only just coming back; and the dog's not much good. – Lady Rose's Daughter by Mrs. Humphry Ward