The phrase "deep depression" refers to a condition of extreme sadness or unhappiness that goes beyond normal levels. It describes a state where a person feels very low, hopeless, and overwhelmed by negative emotions. Full definition
A prominent observation was that some dogs went into deep depression during the first few days of being placed in the refuge. (dogtrainingminneapolis-mn.com)
When I went through a period of deep depression and anger in my life, writing is one of the things that helped pull me through. (redeeminggod.com)
Often it is joined with an exaggerated self - pity or self - accusation and with deep depression about life in general. (religion-online.org)