A brine pool is a volume of brine accumulated in a seafloor depression (sometimes called an underwater, Deepwater bath, or "brine lake"). These pools are deep bodies of water that are three to eight times greater in salinity than the ocean around them. Beneath polar sea ice and in the deep ocean, brine pools are commonly found. Via a process called brine rejection, brine pools below sea ice form.

To increase the salinity gradient, salt is essential for deep-sea brine pools. This salt can come from one of two processes: the dissolution of large salt deposits from tectonic spreading centers by salt tectonics or geothermally heated brine.

High concentrations of hydrogen sulfide and methane are also found in the brine, providing energy for chemosynthetic animals living near the pool. Extremophiles and symbionts are always these animals. Owing to their high salinity and anoxic properties, deep-sea and polar brine pools are harmful to marine animals, which can potentially result in toxic shock and possibly death.

Coupled with their uniquely high salinity, the frequency of brine pool formation has made them a candidate for research on how to use their properties to enhance human science.

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