A brine pool, sometimes called an underwater lake, deepwater or brine lake, is a volume of brine collected in a seafloor depression. These pools are dense bodies of water that have a salinity that is typically three to eight times greater than the surrounding ocean. Brine pools are commonly found below polar sea ice and in the deep ocean. This below-sea ice forms through a process called brine rejection. For deep-sea brine pools, salt is necessary to increase the salinity gradient. The salt can come from one of two processes: the dissolution of large salt deposits through salt tectonics or geothermally-heated brine issued from tectonic spreading centers.