Underwater

Underwater describes the environment and phenomena beneath the surface of a body of water, including oceans, seas, rivers, and lakes. As depth increases, hydrostatic pressure rises by about one atmosphere for every 10 meters of water. Light diminishes with depth and is rapidly absorbed, causing colors to fade; blue and green wavelengths persist longest. Temperature often decreases with depth, producing thermoclines, while salinity and currents vary by location. Because water conducts sound efficiently, underwater acoustics are central to navigation, communication, and sensing.

Biologically, underwater environments host diverse ecosystems, from coastal seagrass beds, kelp forests, and coral reefs to the deep sea. Organisms exhibit adaptations such as gills for extracting oxygen, fins or flippers for propulsion, buoyancy control, and, in some species, bioluminescence. Photosynthesis in sunlight-rich zones supports large communities, while darker zones rely on chemosynthesis or detrital food webs.

Human activity makes extensive use of underwater spaces. Scuba diving and rebreather systems enable direct exploration and work in shallow to moderate depths, while submarines and remotely operated vehicles (ROVs) access deeper regions. Underwater technologies support fields such as marine biology, archaeology, offshore engineering, and salvage.

Environmental pressures threaten underwater habitats, including pollution, overfishing, habitat destruction, and climate-driven changes in temperature and acidity. Sustainable practices, protected areas, and scientific monitoring aim to preserve underwater ecosystems and maintain the services they provide, such as biodiversity, fisheries, and coastal protection.