Brine pools, often nicknamed underwater lakes or lakes of death, are some of the most dangerous environments in the ocean. They are pockets of water so salty and oxygen-free that they form sharp, visible boundaries on the seafloor. Most marine animals instinctively avoid them, but when a fish or crab slips inside, the extreme salinity immediately disrupts its cells, stopping movement and breathing within moments. For scientists, however, these pools provide invaluable insight, offering rare information about extreme chemistry, early Earth conditions, and how life can persist under limits once thought impossible.
What brine pools are, and why they exist at all
Deep-sea brine pools form when extremely salty water collects in depressions on the ocean floor and becomes so dense that it does not mix with the surrounding sea. In the Red Sea, these pools are thought to originate from the dissolution of mineral deposits laid down during the Miocene epoch, between about 23 million and 5.3 million years ago, when sea levels were much lower than today. As salt beds dissolve, the resulting brine sinks and settles into basins, creating isolated lakes within the ocean.The chemistry is extreme. Brine pools can be three to eight times saltier than normal seawater. They are also anoxic, meaning they contain virtually no oxygen. The combined effect of salinity, density and chemistry creates a sharp physical boundary between the pool and the surrounding water, complete with slow-moving waves in complete darkness and a distinct “surface” that can clearly be seen.Globally, scientists know of only a few dozen such pools. They are found in just three regions: the Gulf of Mexico, the Mediterranean Sea and, most prominently, the Red Sea. The deepest known example lies in the Orca Basin in the Gulf of Mexico, about 2,200 metres below sea level. There, a depression roughly 7 by 21 kilometres is filled with brine containing around 300 grams of salt per litre, about eight times saltier than the surrounding gulf.
What happens when animals cross the boundary
For marine animals, brine pools are lethal for reasons that are straightforward and unforgiving. When a fish or crab slips into the brine, the sudden difference in salt concentration triggers rapid osmotic shock. Water is drawn out of the animal’s cells almost immediately. At the same time, the lack of oxygen prevents normal respiration. According to observations made during multiple expeditions, animals that enter the brine are stunned or killed within seconds.Over time, this process creates what researchers describe as underwater graveyards. The floors of many brine pools are littered with the remains of fish, crabs and other organisms that strayed too far. Because scavengers and burrowing animals cannot survive in the anoxic brine, these remains can persist far longer than they would on a normal seabed.Yet the boundary itself has become a hunting ground. During expeditions documented by OceanX, shrimp were observed hovering precisely at the interface between normal seawater and brine. They never cross into the pool. Instead, they wait. When a fish or crab becomes stunned by the brine and drifts back out, the shrimp dart in, seize it and retreat. The brine, in effect, becomes a weapon: a natural trap exploited by predators that have learned exactly where safety ends.
Life without oxygen: microbes that thrive where others die
Despite the hostility of these pools to larger animals, they are anything but lifeless. Brine pools are home to dense communities of extremophile microorganisms, mainly bacteria and archaea, that have adapted to conditions lethal to most forms of life.These microbes do not rely on oxygen or sunlight. Instead, they draw energy from chemical reactions, including the oxidation of sulfur compounds or methane. Their cell walls and membranes are structured to remain stable in extreme salinity and in the presence of toxic chemicals. Thick microbial mats often carpet the edges and floors of brine pools, forming the base of a food web that supports specialised animals at the margins.“At this great depth, there is ordinarily not much life on the seabed,” said Sam Purkis, a marine geoscientist at the University of Miami. “However, the brine pools are a rich oasis. Thick carpets of microbes support a diverse suite of animals.” Among the most striking observations were fish, shrimp and eels that appeared to use the brine itself as part of their hunting strategy, feeding on animals that became incapacitated at the boundary.According to Purkis, these microbial communities are of particular interest because they mirror conditions thought to resemble early Earth. “Our current understanding is that life originated on Earth in the deep sea, almost certainly in anoxic, without oxygen, conditions,” he explained in comments published alongside recent findings. Studying brine pools, he said, offers a glimpse into the kinds of environments in which life first appeared and could inform the search for life on other water-rich worlds.Some of these microbes may also have practical value. Molecules with antibacterial and anticancer properties have previously been isolated from microorganisms living in brine pools, raising the possibility of future medical applications derived from these extreme ecosystems.
The Red Sea’s hidden concentration of brine pools
The Red Sea stands out globally for the number of brine pools it contains. Scientists have identified at least 25 complexes there, more than in any other region on Earth. Until recently, all known Red Sea brine pools were located at least 25 kilometres offshore.That changed with the discovery of the NEOM Brine Pools in the Gulf of Aqaba. First reported in June 2022 in Communications Earth & Environment, the pools were found during a four-week expedition aboardOceanX’s researchvessel OceanXplorer. Using remotely operated vehicles, researchers located the pools at a depth of about 1,770 metres, just two kilometres from the Saudi Arabian coast, a record for proximity to land.The largest of these pools measures roughly 260 metres in length and 70 metres in width, with an area of about 10,000 square metres. Three smaller pools nearby are each less than 10 square metres. Their location so close to shore makes them unique not just biologically, but geologically and historically.
Natural archives sealed by salt
The lack of oxygen in brine pools has another consequence: it preserves sediment layers with unusual clarity. On most seafloors, animals such as worms and shrimp constantly churn the sediment, a process known as bioturbation. In brine pools, those animals cannot survive. As a result, layers of sediment settle and remain undisturbed for centuries.Core samples taken from the NEOM brine pools provide an unbroken environmental record stretching back more than 1,000 years. According to Purkis, these cores capture evidence of past rainfall, floods, earthquakes and tsunamis in the Gulf of Aqaba. Analysis suggests that major floods from intense rainfall occur roughly once every 25 years, while tsunamis strike about once every century.Because the pools lie so close to land, they may also incorporate minerals and material washed in from the coast, effectively recording terrestrial as well as marine events. Researchers have described them as natural time capsules, preserving a layered history of environmental upheavals in a region that is now undergoing rapid coastal development.The findings underscore why brine pools continue to draw scientific attention. They are lethal to most creatures that enter them, yet they sustain unique ecosystems, preserve detailed geological records and offer a rare window into how life can persist under conditions that once dominated the early Earth.
