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.
In addition, major depression episodes may occur before or during persistent depressive disorder â this is sometimes called double depression. Symptoms of persistent depressive disorder can cause significant impairment and may include: Loss of interest in daily activities. Sadness, emptiness or feeling down.