A brine pool is a body of water on the ocean floor so salty and dense that it doesn’t mix with the seawater above it. It sits in a distinct basin, with a visible shoreline and surface, almost like a lake at the bottom of the sea. The salt concentration can reach up to seven times that of normal ocean water, creating a sharp boundary where the dense brine meets the lighter water above.
How Brine Pools Form
Brine pools begin with ancient salt deposits buried beneath the seafloor. Millions of years ago, shallow seas evaporated and left behind thick layers of salt-rich minerals called evaporites. Over geological time, these deposits got buried under sediment. When tectonic shifts crack the seafloor or when water slowly dissolves these underground salt layers, the concentrated brine seeps upward and collects in depressions on the ocean bottom.
Because the resulting water is so much saltier and heavier than regular seawater, it pools in low spots and stays put. The density difference is dramatic enough that the brine forms a distinct surface, complete with ripples and waves if disturbed. Submersible pilots who have visited these sites describe the eerie experience of seeing what looks like a lake, complete with a visible waterline, sitting on the ocean floor thousands of feet below the surface.
Where Brine Pools Are Found
The Red Sea holds more known brine pools than anywhere else on Earth. Their abundance traces back to the region’s geological history: thick evaporite deposits formed during the evolution of the rift basin millions of years ago. Hot brine pools there sit along the mid-ocean ridge at depths typically exceeding 1,000 meters, with most found between 1,500 and 2,850 meters deep. The largest, Atlantis II Deep, has a volume of roughly 17 cubic kilometers and sits at 1,900 to 2,200 meters depth. It is hydrothermally active, reaching temperatures around 68°C, with a pH of 5.3 and extremely high metal content.
Cold brine pools and seeps, often linked to hydrocarbon deposits, appear in the Gulf of Mexico and the Mediterranean Sea. These form in different tectonic settings but follow the same basic principle: subsurface salt dissolving and collecting on the seafloor. In 2020, researchers also discovered the NEOM Brine Pools in the Gulf of Aqaba, consisting of one pool spanning 10,000 square meters and three smaller pools under 10 square meters each. These sit just 2 kilometers from shore, far closer to land than any previously known Red Sea brine pool.
What’s Inside the Water
Brine pools are not just salty. They are cocktails of extreme conditions. Salinity can reach 270 parts per thousand, compared to about 35 for normal ocean water. Hot brine pools, particularly those near hydrothermal vents, run 45 to 69°C warmer than the surrounding deep sea. They tend to be acidic, with pH values around 5.2 to 5.3. Many are completely devoid of oxygen.
Dissolved gases add another layer of hostility. Methane concentrations are high at the boundary where brine meets seawater, and some pools, like the Red Sea’s Kebrit Deep, contain significant levels of hydrogen sulfide, the compound that smells like rotten eggs and is toxic to most animals. The combination of extreme salt, low oxygen, acidity, toxic gases, and sometimes scorching heat makes the interior of a brine pool lethal to virtually any creature that wanders in.
The “Jacuzzi of Despair”
One of the most famous brine pools sits in the Gulf of Mexico, roughly 1,000 meters below the surface. Nicknamed the “Jacuzzi of Despair” by the researchers who explored it, this pool measures about 30 meters in circumference and 3.7 meters deep. The name comes from what happens to animals that stumble into it: crabs, fish, and other creatures that enter the dense, toxic brine become paralyzed and eventually die, their bodies preserved in the hyper-salty water like specimens in a jar. The pool’s warmth, combined with its lethality, gives it the unsettling appearance of a hot tub littered with pickled remains.
Life at the Edge
While the interior of a brine pool is deadly, the boundary layer where brine meets normal seawater is one of the most biologically interesting zones in the deep ocean. This thin interface creates a gradient of salt, temperature, oxygen, and chemicals that certain organisms have evolved to exploit.
Bacteria and archaea dominate these transition zones. Researchers studying Red Sea brine pools have found communities of microbes from groups including Proteobacteria, Bacteroidetes, Firmicutes, and the archaeal phylum Euryarchaeota living across multiple brine sites. These organisms survive through specialized adaptations. Some produce a compound called ectoine, a stress molecule that helps cells cope with extreme salt by essentially forcing water to stay inside the cell rather than being drawn out. Others modify their cell membranes with specific fatty acids and terpenes to maintain flexibility under crushing pressure and extreme temperatures.
At some brine pool edges, the high methane concentration fuels a process called chemosynthesis, where microbes harvest energy from chemical reactions rather than sunlight. Methane-eating bacteria thrive at the brine-seawater interface, and their productivity supports small ecosystems of mussels and other invertebrates that cluster around the pool’s rim. In the Gulf of Mexico, dense colonies of mussels and tube worms ring certain brine pools, feeding on or partnering with chemosynthetic bacteria. These oases of life in the deep ocean exist entirely independent of the sun.
Why Scientists Study Them
Brine pools attract attention from two very different scientific communities. Biochemists see them as treasure chests of unusual enzymes. Microbes that function in extreme salt, heat, and acidity produce proteins with industrial potential. Enzymes isolated from Atlantis II Deep, for example, remain stable at 70°C and function in salt concentrations that would destroy most biological molecules. These properties make them candidates for applications in manufacturing, pharmaceuticals, and chemical processing where harsh conditions are the norm.
Astrobiologists, meanwhile, view brine pools as some of the best models on Earth for what might exist on other worlds. Several moons in the outer solar system, particularly Jupiter’s Europa and Saturn’s Enceladus, are believed to harbor oceans beneath their icy surfaces. Those oceans are expected to be briny, under high pressure, and potentially influenced by hydrothermal activity. Deep-sea brine pools check all of those boxes. The fact that microbial life not only survives but thrives at the edges of these pools, powered by chemical energy rather than sunlight, is one of the strongest arguments that life could exist in similar environments elsewhere in the solar system. Researchers have specifically proposed Red Sea and Mediterranean brine pools as terrestrial analogs for future exploration of these extraterrestrial oceans.