The Pacific Ocean is the largest ocean in the world at 155,557,000 sq km.   The Atlantic Ocean is the second largest ocean in the world at 76,762,000 sq km.
The cause of major depressive disorder is a combination of brain chemistry, family history, and psychosocial environment. It is not certain which of these factors dominates, but abnormal levels of the neurotransmitters norepinephrine, serotonin, and dopamine are closely linked with depression.
By David Joel Miller. Major Depressive Disorder is a specific diagnosable disorder listed in the DSM (Diagnostic and Statistical Manual of Mental Disorders.) Sometimes we use the term imprecisely to refer to both the common sense feeling of sadness and a series of specific mental disorders that we professionals call mood disorders.