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| Pelagic | |
| Photic | |
| Epipelagic | |
| Aphotic | |
| Mesopelagic | |
| Bathyalpelagic | |
| Abyssopelagic | |
| Hadopelagic | |
| Demersal | |
| Benthic | |
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| Pycnocline | |
| Isopycnal | |
| Chemocline | |
| Halocline | |
| Thermocline | |
| Thermohaline | |
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The abyssal zone is the abyssopelagic layer or pelagic zone that contains the very deep benthic communities near the bottom of oceans. "Abyss" derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 4,000 to 6,000 metres (13,123 to 19,685 feet), this zone remains in perpetual darkness and never receives daylight. It is the deeper part of the midnight zone which starts in the bathypelagic waters above.[1][2]
Its permanent inhabitants (for example, the black swallower, tripod fish, deep-sea anglerfish and the giant squid) are able to withstand the immense pressures of the ocean depths, up to 76 megapascals (11,000 psi).[1] Many abyssal creatures have underslung jaws to sift through the sand to catch food.[2] The deep trenches or fissures that plunge down thousands of meters (feet) below the ocean floor (for example, the midoceanic trenches such as the Mariana Trench in the Pacific) are almost unexplored.[1] Only the bathyscaphe Trieste, the remote control submarine Kaiko and the Nereus have been able to descend to these depths.[3][4] These regions are also characterized by continuous cold and lack of nutrients. The abyssal zone has temperatures around 2°C to 3°C (35°F to 37°F) through the large majority of its mass.[2]
The area below the abyssal zone is the sparsely inhabited Hadal zone.[5] The zone above is the bathyal zone.[5] These three zones belong to the deep-sea realm. Above on the continental platform there are respectively the euphotic and dysphotic zones. The abyssal zone lies partially in the dysphotic and partially in the aphotic zones.[6]