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.
Three main types of depressive disordersâmajor depression, persistent depressive disorder, and bipolar disorderâcan occur with any of the anxiety disorders. Major depression involves at least five of these symptoms for a two-week period.any people who develop depression have a history of an anxiety disorder earlier in life. There is no evidence one disorder causes the other, but there is clear evidence that many people suffer from both disorders. Read more information about depression from the National Institute of Mental Health.