Relating to the deepest parts of the ocean, typically 4,000 to 6,000 meters deep, where no sunlight penetrates and conditions are extremely cold and high-pressure. The abyssal zone covers most of the ocean floor.
From Greek 'abyssos' meaning bottomless or unfathomed depth, from 'a-' (without) and 'byssos' (bottom). The term entered oceanographic vocabulary in the 19th century as deep-sea exploration revealed the vast extent of these ultra-deep ocean regions.
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