The ocean is divided vertically into distinct zones based on how much sunlight reaches them. The uppermost layer is the epipelagic, or sunlight, zone, extending from the surface down to about 200 meters; this is where the majority of marine photosynthesis occurs and where most of the ocean's familiar life is concentrated. Below it lies the mesopelagic, or twilight, zone, from roughly 200 to 1,000 meters deep, where some light penetrates but not enough for photosynthesis. The bathypelagic, or midnight, zone, between about 1,000 and 4,000 meters, is completely dark and populated largely by bioluminescent organisms that produce their own light.
Even deeper, the abyssopelagic zone stretches from about 4,000 to 6,000 meters, with near-freezing temperatures and crushing pressure. The hadopelagic zone lies below 6,000 meters, found in deep ocean trenches such as the Mariana Trench, where the Challenger Deep reaches roughly 10,935 meters below sea level. These names reflect the harsh conditions: hadopelagic comes from Hades, the Greek god of the underworld. Despite the extreme environment, life thrives in surprising places — hydrothermal vents along ocean ridges, for example, are fissures where geothermally heated water erupts and supports entire ecosystems through chemosynthesis rather than photosynthesis.
Coral reefs are among the most productive marine ecosystems. They are built up over centuries by colonies of tiny coral polyps, which secrete calcium carbonate (CaCO₃) to form hard skeletons. The three main reef types are fringing reefs, which grow close to the shore; barrier reefs, which are separated from the coast by a lagoon; and atolls, ring-shaped reefs encircling a central lagoon. The Great Barrier Reef off the northeast coast of Australia is the largest coral reef system in the world, stretching more than 2,300 kilometers, while the Mesoamerican Barrier Reef along the coasts of Mexico, Belize, Guatemala, and Honduras is the second-largest. When corals become stressed — most often by rising water temperatures — they expel their symbiotic zooxanthellae algae in a process known as coral bleaching, turning white and risking death.
Coastal and nearshore environments also host unique habitats. Estuaries are partially enclosed coastal bodies of water where freshwater from rivers mixes with saltwater from the ocean, creating exceptionally productive nurseries for fish and other wildlife. Fjords are long, narrow, deep inlets of the sea formed by glacial erosion and flanked by steep cliffs; famous examples are found in Norway and New Zealand. These varied zones — from sunlit surface waters to crushing deep trenches, from coral gardens to glacial inlets — illustrate just how many distinct ecosystems coexist within the global ocean.