Extremophiles are living organisms that can thrive in environmental conditions that are inhospitable to most lifeforms. These species may be animals, plants or bacteria; the unusual conditions in which they may persist can involve extreme salinity, acidity, temperature, radiation or pressure. Such occurrences may take place in such unusual climate regimes as Antarctica, or in such unusual geographic features as hypersaline lakes.

They may also be found at extreme ocean depths where high pressures and unusual chemical environments, such as near hydrothermal ocean vents, are encountered. The image to the right shows tubeworms found near such hydrothermal vents.