For thousands of years, humans have flocked to the Dead Sea to immerse themselves in one of the saltiest bodies of water in the world.
Amazingly enough, the Dead Sea – which is actually a hypersaline lake cut off from the ocean waters of the Mediterranean Sea – is actually getting saltier with time, even though it's already nearly 10 times saltier than seawater.
Mostly this transition is driven by human activity, with freshwater from the Jordan River – which traditionally fed the Dead Sea – being diverted in recent decades for other purposes in the region, including agriculture, mining, and drinking water.
With that freshwater no longer coming in, the salty Dead Sea's water content is slowly evaporating away, leaving only crusty salt crystals in its wake.
But there's something about those salt crystals – and the way they seem to move within what remains of the Dead Sea – that has confused scientists for decades.
Specifically, after freshwater started being diverted away from the hypersaline lake, researchers observed that salt crystals were precipitating out of the top layer of water, almost as if they were snowing down through the water, and piling up at the bottom of the lake.
This phenomenon in general is called salt fingering – but scientists didn't understand how it was taking place in the Dead Sea, since it's not known to take place in any other hypersaline water bodies.
"Initially you form these tiny fingers that are too small to observe… but quickly they interact with each other as they move down, and form larger and larger structures," says mechanical engineer Raphael Ouillon from the University of California Santa Barbara.
In new research, Ouillon and his team studied this 'snowing' phenomenon in the Dead Sea, and figured out what makes it happen.
When sunlight shines upon the Dead Sea, the top layer of the lake gets warmer than the cooler waters underneath, and water evaporates at this heated surface, meaning the top layer of the water becomes the saltiest overall.
So how do the salt crystals produced by evaporation begin their journey down into the depths of the Dead Sea? In 2016, some of the members proposed a possible explanation, and the new research is the first test of this hypothesis.
Before now, what researchers didn't understand about salt fingering in the Dead Sea was how salt could become displaced from the top layer of the lake, since the warm and cooler waters ordinarily wouldn't mix easily.
In the new research, Ouillon and his team simulated how it could be possible, if waves or other sources of motion could push small parcels of warm water downwards to mix with the cooler water.
When this happens, the heated water gets cooled, which makes it offload its salt content; that content then begins its snow-like descent to the bottom of the lake.
"The net result is a strong, downward salinity flux that leads to the undersaturation of the epilimnion [upper layer], while the hypolimnion [lower, cooler layer] becomes oversaturated and precipitates halite," the authors write, noting that the same phenomenon might also explain the buildup of other massive salt formations found in the geological record.
"We know that many places around the world have thick salt deposits in the Earth's crust, and these deposits can be up to a kilometre thick," one of the team, fluid dynamics researcher Eckart Meiburg, explains.
"But we're uncertain how these salt deposits were generated throughout geological history."
Now, we're one step closer to understanding why, thanks to the Dead Sea - which sadly may not be long for this world.
The findings are reported in Water Resources Research.