You can find a lot of weird things when you dive deep into the ocean: bioluminescent fish, microbes in 86 million-year-old clay, or a priceless blue diamond necklace casually chucked from a research boat.

But one thing you might not expect to find at the bottom of the ocean… is a lake. In the Gulf of Mexico, there’s a lake so deadly that anything that goes for a swim gets pickled.

Yet there’s a thriving ecosystem literally living on the edge, which might give astrobiologists a hint at how life could thrive on other worlds. Its name? The Jacuzzi of Despair.

A kilometer below the ocean’s surface, not too far off the coast of Louisiana, this underwater lake was discovered back in 2013 using the Remotely Operated Vehicle called Hercules. The “Jacuzzi”, or sometimes “Hot Tub”, moniker is because it’s warmer than the nearby 4-degree Celsius ocean water. During return visits in 2015, researchers recorded temperatures from 7.8 up to 19 degrees.

This underwater lake, and others like it around the world, are scientifically known as brine pools. Brine pools form over millions of years. To make our Jacuzzi, a proto-Gulf of Mexico got closed off from the rest of the ocean during the Jurassic period, some 165 million years ago.

Over time, the water evaporated, leaving salts and other minerals behind. Then, that salt bed got buried under layers of sediment, and eventually was submerged under water again. Under all the weight of the sediment and ocean water, the salt bed deformed and shifted around.

Cracks in the sediment allowed water to creep downward, dissolving the salt to form a super concentrated brine. In the Jacuzzi of Despair, it’s roughly 4 times saltier than the surrounding ocean. The brine was squeezed upwards by the salt tectonics and some gases that were also trapped beneath the sediment layers.

And since the salty brine is much denser than regular seawater, it pooled up on the ocean floor. Because the Jacuzzi of Despair contains some hydrocarbon gas leakage, it’s also considered to be a cold seep — a name that just comes from the fact that it’s cold relative to hydrothermal vents, which can reach temperatures over 400 degrees Celsius! Specifically, the Jacuzzi is saturated with methane, and there’s also non-hydrocarbon gases like the very toxic hydrogen sulfide.

But one gas you won’t really find in that super salty water is oxygen. What little there was in the brine pool to begin with got used up in chemical reactions, like organic stuff decomposing. And oxygen doesn’t get replenished, because the huge difference in density at the pool’s surface keeps gases from the nearby seawater from entering the brine.

The anoxia and other toxic gases makes the Jacuzzi of Despair deadly to most life except extremophile bacteria and archaea. Like, there’s some chemosynthetic bacteria that use chemical reactions with the sulfur in the pool to make the energy they need to live, instead of sunlight. But the unlucky crabs or fish that venture into the Jacuzzi of Despair don’t just die.

Because of the brine’s high salt content, they’re effectively pickled — the water sucked from their cells in a doomed effort to balance the concentration of salt inside and outside their bodies. And yet there are complex organisms making a life down there. The Jacuzzi of Despair is surrounded by 3-meter-high walls, built from precipitated minerals from the brine, like barite.

All along these walls live massive colonies of mussels with chemosynthetic bacteria in their gills that can use the methane from the pool as a source of energy and of carbon. You’ll also find shrimp, crabs, and amphipods crawling around down there. It’s not the most diverse ecosystem on the planet, but these creatures are on the edge of instant death, so maybe give them a break.

Besides telling us about our own planet, studying extreme underwater places like brine pools can also help us figure out how life might thrive on other water worlds — objects with liquid water in our solar system and beyond. Saturn’s moon Enceladus, for instance, is probably the best candidate for extraterrestrial life that we’ve found so far. In April 2017, the Cassini mission detected hydrogen gas in the moon’s southern water plumes, coming from what researchers think is a big subsurface lake.

This hydrogen gas, along with carbon dioxide, could be used as a food source by chemosynthetic life that makes methane as a byproduct. And any methane could be used by other kinds of microbes, like the bacteria living inside the mussels near the Jacuzzi of Despair. Because the Cassini mission is about to end, we may have to wait decades to get more definitive answers.

Until then, we can study more brine pools to get a peek at some of the strangest life on Earth — and maybe predict what else could survive in extreme environments throughout the universe. Thanks for watching this episode of SciShow! If you want to learn more about extreme non-Earth environments, check out our sister channel SciShow Space at youtube.com/scishowspace.