There's a Lake at the Bottom of the Ocean, and It Kills Almost Everything That Swims In

There’s a lake at the bottom of the ocean, with a shoreline and a surface of its own, sitting on the sea floor thousands of feet down. Fish that wander in go into shock, and crabs that crawl in often never crawl back out; their bodies just float there, pickled. Here’s how a lake can exist underwater, why it kills almost everything that touches it, and why scientists now treat these deadly pools as time capsules and maybe even clues to life on other worlds.

The Jacuzzi of Despair

More than 3,000 feet down in the Gulf of Mexico, it’s cold and pitch dark, and the sea floor is mostly mud.

Then the lights of a robot submarine sweep across something strange: a raised rim about 10 feet tall, like the edge of a small crater. Inside sits a pool of glassy water that, on one side, spills over the rim and flows down the slope like a slow, ghostly waterfall.

Scientists exploring with the research ship Nautilus found this pool. It’s only about 12 feet deep, but the water inside is four to five times saltier than the surrounding ocean. Bubbles of methane rise through it, which earned it a nickname: the Jacuzzi of Despair. It’s even a little warmer than the water around it.

The despair part is the bodies. Crabs and other small creatures lie scattered through the pool, having wandered in and never come out. As one scientist put it, they die, and then they get pickled and preserved.

How a lake forms inside the ocean

Water inside water shouldn’t be able to form a separate lake. Pour warm water into a cold bath, and a few minutes later it’s all mixed.

Salt changes that. Very salty water, called brine, is much heavier than normal seawater. It sinks to the lowest spot it can find and stays there, like syrup at the bottom of a glass, while the rest of the ocean sits on top.

More than 160 million years ago, in the age of the dinosaurs, the Gulf of Mexico was a shallow sea that got cut off from the ocean and slowly dried up. As the water evaporated, it left its salt behind, layer after layer, until in places the salt was up to five miles thick. That buried salt layer still stretches underground all the way to southern Arkansas.

Mud and sand carried in by rivers buried it, but salt is a strange kind of rock. Under enough weight it slowly flows, a bit like squeezed toothpaste. Over millions of years, giant blobs of salt pushed back up toward the sea floor. Where seawater touches that salt, it dissolves it into brine, which oozes out and collects in the lowest dips.

The monster pool

Most brine pools are small; some are smaller than a bathtub. But about 200 miles southwest of New Orleans, more than 7,000 feet down, lies a giant: the Orca Basin.

Its brine lake covers about 47 square miles, roughly twice the size of Manhattan. In places it’s more than 700 feet deep, and it’s around eight times saltier than seawater. Every year the ocean dissolves about half a million tons of that dinosaur-age salt into it.

The basin’s walls are steep, and underwater landslides have happened there. A 2019 computer-modeling study found that a big landslide could set off waves inside the brine lake hundreds of feet tall, the biggest up to around 1,200 feet. That’s taller than the Eiffel Tower, though it’s a model result, not something anyone has seen.

Waves that size could slosh deadly brine over the rim and onto the neighboring sea floor, wiping out the life there.

Why brine kills

Brine is a triple threat. First, there’s almost no oxygen: step past the surface of the brine, and it’s basically gone. Second, it can be loaded with toxic chemicals such as hydrogen sulfide. Third, all that salt pulls the water right out of an animal’s body.

A fish that swims in goes into shock almost instantly. Because almost nothing that normally breaks bodies down can survive in the brine, the dead don’t rot the way they should. They can stay there for years, like a museum of the unlucky.

Some animals exploit this. In 2020, scientists found a brand-new set of brine pools in the Gulf of Aqaba, at the northern tip of the Red Sea. Around their edges, fish, shrimp and eels seemed to lurk, ready to snap up any creature stunned by the brine.

Life on the shore

Right at the edge of these pools, life is booming. Around one brine pool in the Gulf of Mexico lies a beach made of mussels, covering an area about the size of two tennis courts.

How do they survive without sunlight? Bacteria living inside the mussels’ gills eat methane, the same gas seeping from the brine, and turn it into food they share with their host.

Tube worms live here too, and inside the brine itself tough salt-loving microbes thrive where almost nothing else can. Some form reddish patches that hover right where the brine meets the seawater.

In the Mediterranean, scientists reported something that sounded impossible: animals so small you’d need a microscope to see them, living in the mud beneath a brine lake with no oxygen at all. Other researchers aren’t convinced those animals were really alive down there, and the question still isn’t settled.

Icicles of death

Brine doesn’t only kill in the deep. When Antarctic sea ice forms, it pushes its salt out. In winter the air above the ice can be colder than −4°F, while the sea below sits just under freezing. The leftover brine, extremely cold and salty, sinks, and as it does it freezes the seawater around it into a hollow tube of ice that reaches toward the sea floor like a frozen finger.

In 2011, a BBC crew filmed one of these brinicles forming for the first time. When it touched the bottom, a web of ice spread across the sea floor, freezing the starfish and sea urchins in its path. Brine that keeps sinking beneath sea ice can also collect in hollows on the bottom, making brine pools in polar seas too.

Time capsules in the Red Sea

The Gulf of Aqaba pools lie more than a mile deep, and the team almost missed them: they didn’t show up on the first maps of the sea floor. One scientist had a hunch, and the team finally spotted them during the last five minutes of a 10-hour dive.

They’re also only about a mile and a quarter from shore. Because no animals live in the brine to dig through the mud, the layers that settle on the bottom stay undisturbed.

Those layers work like the pages of a history book. In them, scientists can read the flash floods, earthquakes and tsunamis that hit this coast, going back thousands of years. That can help predict what the coast might face next.

Why it matters

Scientists study the microbes that survive in brine to understand where life could exist elsewhere, such as the salty ocean thought to lie beneath the ice of Jupiter’s moon Europa. Some brine-loving microbes also make chemicals that researchers are studying as possible new medicines.

So a lake at the bottom of the sea, one that kills almost everything it touches, might also help us find life on other worlds and protect people on this one.

Sources

• NOAA Ocean Explorer: Gulf of Mexico brine pools, Brine Pool NR1 and the Louann Salt
• The Christian Science Monitor (2016), on the Oceanography study of the “Jacuzzi of Despair”, quoting Scott Wankel (Woods Hole Oceanographic Institution)
• E/V Nautilus (Nautilus Live) and ROV Hercules expeditions (Erik Cordes, Temple University)
• Sawyer et al. (2019), Scientific Reports, and Eos (2019): the Orca Basin and its modeled brine waves
• AAPG Bulletin (2007): Orca Basin dimensions
• Purkis et al. (2022), Communications Earth & Environment: the NEOM brine pools in the Gulf of Aqaba
• University of Miami (2022) and Live Science (2022): reporting on the Gulf of Aqaba pools
• Danovaro et al. (2010, 2016) and Bernhard et al. (2015), BMC Biology: the L’Atalante basin Loricifera debate
• BBC Frozen Planet (2011) and Mark Brandon (The Open University): the first filmed brinicle