The Collaborative International Dictionary of English v.0.48:

Deepen \Deep"en\, v. t. [imp. & p. p. Deepened; p. pr. & vb.
   n. Deepening.]
   1. To make deep or deeper; to increase the depth of; to sink
      lower; as, to deepen a well or a channel.
      [1913 Webster]

            It would . . . deepen the bed of the Tiber.
                                                  --Addison.
      [1913 Webster]

   2. To make darker or more intense; to darken; as, the event
      deepened the prevailing gloom.
      [1913 Webster]

            You must deepen your colors.          --Peacham.
      [1913 Webster]

   3. To make more poignant or affecting; to increase in degree;
      as, to deepen grief or sorrow.
      [1913 Webster]

   4. To make more grave or low in tone; as, to deepen the tones
      of an organ.
      [1913 Webster]

            Deepens the murmur of the falling floods. --Pope.
      [1913 Webster]

WordNet (r) 3.0 (2006):

deepening
    adj 1: accumulating and becoming more intense; "the deepening
           gloom"; "felt a deepening love"; "the thickening dusk"
           [syn: deepening(a), thickening(a)]
    n 1: a process of becoming deeper and more profound

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "deepening":
   accelerando, acceleration, aggravation, amplification, annoyance,
   augmentation, beefing-up, blowing up, blowup, concentration,
   condensation, consolidation, contentiousness,
   deliberate aggravation, depression, deterioration, digging,
   drilling, embittering, embitterment, enhancement, enlargement,
   exacerbation, exaggeration, exasperation, excavation, explosion,
   heating-up, heightening, increase, information explosion,
   intensification, irritation, lowering, magnification, mining,
   pickup, population explosion, probing, provocation, redoubling,
   reinforcement, sharpening, sinkage, sinking, souring, speedup,
   step-up, strengthening, tightening, tunneling, worsening