Deep sea
The deep sea is the ocean below the sunlit surface layer, where darkness, cold, and high pressure shape ecosystems and physical processes.
Hydrostatic pressure: Pressure exerted by a fluid at rest, increasing with depth under the weight of overlying water. Its steep increase with depth shapes organisms, equipment, and the limits of exploration.
Epipelagic zone: The upper ocean layer, extending from the surface to roughly 200 meters, where sunlight supports photosynthesis. It marks the sunlit boundary above the deep sea.
Deep-sea submersible: A crewed underwater vehicle designed to descend to great ocean depths. Submersibles carry observers to habitats beyond the reach of ordinary diving.
Hydrothermal vent: A seafloor opening that releases geothermally heated, mineral-rich water. Vents support dense communities powered by chemical energy rather than sunlight.
Deep-sea ecology: The study of interactions among organisms and environments in deep ocean habitats. It frames unresolved questions about food webs, biodiversity, and ecosystem function.
Marine snow: A continuous fall of organic particles from upper waters toward the seafloor. This drifting material supplies much of the deep sea’s food.
Mesopelagic zone: The twilight ocean layer, roughly 200 to 1,000 meters deep, with too little light for photosynthesis. It is the upper transition into deep-sea conditions and hosts daily vertical migrations.
Remotely operated underwater vehicle: An uncrewed underwater vehicle controlled from a surface vessel through a tether. ROVs collect imagery and samples while operating under crushing pressure.
Cold seep: A seafloor site where methane, sulfide, or other hydrocarbon-rich fluids seep into the ocean. Seeps sustain specialized communities using chemicals released from sediments.
Deep-sea biodiversity: The variety of life forms and ecological communities found in deep ocean environments. Many deep-sea species and their distributions remain undocumented.