Thermophiles, a type of extremophile, produce some of the bright colors of Grand Prismatic Spring, Yellowstone National Park
An **extremophile** is an organism (a living thing) which lives best in extreme conditions that are harmful to most life on Earth. They are different from organisms that live in normal places, called *mesophiles* or *neutrophiles*.

In the 1980s and 1990s, biologists found that microbes can survive in extreme environments. These are niches that are extreme in some way. They may be extremely hot, or cold, or dry, or under huge pressure, or very salty or acidic. Complex organisms, such as animals or plants, cannot live in these environments.

Some scientists suggest that life may have begun on Earth in hydrothermal vents far below the ocean surface. Environments such as hot oceans, hot springs and deep ocean hydrothermal vents would have been common during the Archaean eon, about 3.9 billion years ago. Early forms of life lived in those conditions.

Types of extremophiles
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Most known extremophiles are microbes. The domain Archaea has well-known examples of extremophiles, but some bacteria are also extremophiles. It is a mistake to use the term extremophile for all archaeans, since some of them are mesophilic. Not all extremophiles are unicellular: some extreme environments have animals which are multicellular. 

Some extremophiles fall under several categories. For example, organisms living inside hot rocks deep under Earth's surface are both *thermophilic* and *barophilic*.

AcidophilesAn organism that grows best at pH levels of 3 or below.
AlkaliphilesAn organism that grows best at pH levels of 9 or above
EndolithsAn organism that lives in microscopic spaces within rocks, fissures, aquifers, and faults filled with groundwater deep under the ground.
HalophilesAn organism that needs a high concentration of salt to grow.
HyperthermophileAn organism that can live well at temperatures between 80–122 °C, as in hydrothermal vents in the deep ocean.
HypolithsAn organism that lives inside rocks in cold deserts.
LithoautotrophsAn organism (usually bacteria) that only gets carbon from carbon dioxide and inorganic oxidation. They are chemolithotrophs, such as *Nitrosomonas europaea*. These organisms get energy from mineral compounds such as iron pyrites. They act in geochemical cycling and wearing away of bedrock to form soil.
MetalotolerantThese organisms are not harmed by high levels of dissolved heavy metals in solution, such as copper, cadmium, arsenic, and zinc.
OligotrophAn organism that can grow in environments with very few nutrients.
OsmophileAn organism that can grow in environments with lots of sugar.
PiezophileAn organism that lives best under high pressure, such as deep inside the Earth's surface and in deep ocean trenches.
PolyextremophileAn organism that is in more than one category of extremophiles.
Psychrophile/CryophileAn organism that grows better at temperatures of 15 °C or lower. They are common in cold soils, permafrost, polar ice, cold ocean water, and under snow in high mountains.
RadioresistantOrganisms that can live with high levels of ionizing radiation. Usually this refers to ultraviolet radiation, but there are also some organisms which can repair damage cause by gamma radiation.
ThermophileAn organism that can live well at temperatures between 60–80 °C.
ThermoacidophileAn organism that is both a *thermophile* and an *acidophile*. It grows best in temperatures of 70–80 °C and pH between 2 and 3.
XerophileAn organism that can grow in extremely dry places, such as the Atacama Desert.