Hadal zone

The hadal zone is the ocean region below about 6,000 meters, found chiefly in deep-sea trenches. It includes the ocean’s greatest depths, where pressure is extreme and sunlight is absent.

Hydrostatic pressure: Pressure exerted by a fluid at rest, increasing with depth beneath its surface. At hadal depths, immense water-column pressure constrains organisms and equipment.
Oceanic zone: The open-ocean region beyond the coastal waters of a continent or island. The hadal zone is the deepest part of the oceanic environment.
Challenger Deep: The deepest known point in the ocean, located in the Mariana Trench. It marks the greatest measured depth within the hadal zone.
Bathyscaphe: A self-propelled deep-sea submersible designed to descend through water using a buoyant float. Bathyscaphes made direct exploration of hadal depths possible.
Hadal ecology: The study of organisms and ecological processes in ocean depths below about 6,000 meters. Sparse observations leave food webs and population patterns poorly characterized.
Deep-sea trench: A long, narrow depression in the seafloor, typically formed at a convergent plate boundary. Trenches contain most hadal seafloor and create its deepest habitats.
Bathymetry: The measurement and mapping of underwater depth and seafloor topography. Mapping reveals the trenches that define the hadal zone’s geography.
Mariana Trench: A deep-sea trench in the western Pacific Ocean containing the Challenger Deep. Its deepest basin is the best-known example of hadal habitat.
ROV KAIKO: A Japanese remotely operated vehicle that reached the Challenger Deep in 1995. KAIKO enabled sampling and observation at hadal depths without a crewed descent.
Deep-sea gigantism: The tendency of some deep-sea animals to grow larger than related shallow-water species. Hadal species raise questions about how pressure, food, and temperature shape body size.

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