Dictionary:

deepen

  (dē'pən)

To make or become deep or deeper.

verb

To make greater in intensity or severity: aggravate, enhance, heighten, intensify, redouble. See increase/decrease.

Definition: make depth greater
Antonyms: fill

v

Definition: make more intense
Antonyms: lighten, pale, quieten, soften

The verb has 4 meanings:

Meaning #1: make more intense, stronger, or more marked
  Synonyms: intensify, compound, heighten

Meaning #2: become more intense
  Synonym: intensify

Meaning #3: make deeper

Meaning #4: become deeper in tone
  Synonym: change