Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen.  The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. Anoxic waters can be contrasted with hypoxic waters, which are low (but not lacking) in dissolved oxygen.  This condition is generally found in areas that have restricted water exchange.
In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier, as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin.  Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen.
Anoxic waters are a natural phenomenon, and have occurred throughout geological history. The Permian–Triassic extinction event, a mass extinction of species from the worlds oceans, may have resulted from widespread anoxic conditions combined with ocean acidification driven by a massive release of carbon dioxide into Earths atmosphere.  Many lakes have a permanent or temporary anoxic layer created by respiration depleting oxygen at depth and thermal stratification preventing its resupply.
Anoxic basins exist in the Baltic Sea, the Black Sea, the Cariaco Trench, various fjord valleys, and elsewhere. Eutrophication has likely increased the extent of anoxic zones in areas including the Baltic Sea, the Gulf of Mexico, and Hood Canal in Washington State.