The azoic zone is the part of the deep sea thought lifeless in the mid-19th century.
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It was thought that the abyss was filled with a thick layer of 4oC (since seawater was thought to be densest at that temperature), motionless water which, combined with the tremendous pressures and absence of sunlight, virtually guaranteed an absence of life. The term was coined by the naturalist Edward Forbes in the 1840s who, after dredging for life forms in various regions, postulated eight bands or depth zones, each characterized by a particular assemblage of animals. These zones extended to a lower limit he set at about 300 fathoms below which the existence of life was highly unlikely. His results (and therefore perceptions) on this issue were skewed by an 1841 cruise in the eastern Mediterranean Sea where he dredged for lifeforms at depths up to 230 fathoms in what is now known to be a relatively barren area. The contrast of this with the rich hauls he made in shallower waters around England led to his thinking the abyss devoid of life.
Physical Oceanography Index
Susan Schlee. The Edge of an Unfamiliar World: A History of Oceanography. E. P. Dutton & Co., Inc., 1973.
(2011). Azoic zone. Retrieved from http://www.eoearth.org/view/article/51cbed067896bb431f68f1a2