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.
Some common triggers or causes of major depression include: 1  Loss of a loved one through death, divorce, or separation. 2  Social isolation or feelings of being deprived. 3  Major life changes -- moving, graduation, job change, retirement.  Personal conflicts in relationships, either with a significant other or a superior.