If you’ve ever been swimming in the ocean, you know a good chunk of the experience  is being force fed gallons of saltwater.

You come out feeling like a  crusty, over-salted fish and you're desperate for a sip  of good, clean, fresh water. Well multiply that experience by 10 and you get one of the saltiest bodies of water  on

Earth: Utah’s Great Salt Lake. It’s so salty they had to change its name! It’s so salty it forms crystals! It’s  so salty that half of it turned purple!

In fact, until the 1800s, people  thought the only way to explain the salt was a secret underground tunnel  connecting it to the Pacific Ocean. Today, that idea doesn't hold water. But the actual story is almost as strange.

See, thirty thousand years  ago, the Great Salt Lake took up a quarter of Utah  and was full of freshwater. But then, it went through  a dramatic transformation. We know this, in large part,  because of a pioneering scientist who had a little help from  some very small friends. [♪ INTRO] In the 1930s, Ruth Patrick set off  into the water of the Great Salt Lake.

She carried a long pole with a  pointed metal tube on the end, which she plunged deep into the  muddy peat at the bottom of the lake. When she pulled it back up, the tube was  filled with a sample of blueish grey mud. Inside this mud were the treasures she was after.

Patrick was collecting the remnants of  tiny single celled algae called diatoms. These otherworldly looking little  guys have a hard silica cell wall called a frustule that you can only  truly appreciate under a microscope. She was collecting them because  the hard silica of diatom frustules preserves really well in sediments  at the bottom of lakes and oceans.

Yes, they can live in both fresh and saltwater. But every species of diatom has a specific saltiness level that they’re usually found in. And Patrick knew that she  could use them to prove that tens of thousands of years ago, the  Great Salt Lake was full of freshwater.

While most of her samples alternated  between fresh and salty diatom species, the deepest and oldest samples  had almost all freshwater diatoms. So that was like a totally different lake. In fact, back when those diatoms  were alive, the Great Salt Lake was called Lake Bonneville, and it  was nearly as big as Lake Michigan.

But around 15,000 years ago, so not that long ago, a wall of the lake eroded and collapsed, leading to what scientists think was one  of the biggest floods in Earth’s history. 241 Olympic sized swimming pools  drained from the lake every second. That might sound too big to be true,  but the evidence is everywhere. Over time, layers of mud and sediment  accumulated at the bottom of the lake.

When researchers drilled  down through these layers, they found one where the sediment suddenly changed colour as more  carbonate minerals appeared. They think that abrupt change  was the moment of the flood. You can still see where all the water went.

It carved a path through Red Rock Pass  and the Snake River Plain in Idaho. Afterwards, more water evaporated  during a series of droughts and the lake shrank even more. In fact, scientists can  see this decline by looking at which sediments contain diatoms  that tend to live in deep, dark water.

Those diatoms tell us that  as the water level declined, light penetrated through more of  the lake and this habitat was lost. By around 13,000 years ago,  the lake became something similar to what we see today:  10 meters at its deepest point, a lot less than the 300 meters  Lake Bonneville reached. And as the water in the lake went  down, the salt concentration went up.

See, the Great Salt Lake has  three rivers flowing into it, all of them always carrying a little bit  of salt, and then no rivers flowing out. So a lot of that water evaporates,  but the salt can’t evaporate, so it just sits there, accumulating. And the concentration of salt in  the North half of the lake has gotten a whole lot higher thanks to,  if you can believe it, us humans.

But before I tell you about that,  a quick word from our sponsors. Thanks to JMP for supporting this SciShow video! JMP is a statistical analysis software  designed to help you with powerful analytics.

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The advanced modeling and statistics  suite in JMP means researchers can use the software from the variable screening  stage all the way through data analysis. They offer all that with a  30-day free trial for anyone, anywhere at jmp.com/scishow. In 1902, the Southern Pacific Railroad  Company wanted to cut down the time that it took to send trains  all the way around the lake.

So they built a 19 kilometer wooden  trestle bridge right across it. But by the 1940s, the wood trestle needed repairs and upgrades to support heavier trains. Hoping for a more robust solution,  the railroad company built a solid rockfill causeway in 1959.

But unlike the wood beams  that made up the trestle, the piled up rocks mostly stopped  water from flowing through them. So, from that point on, the Great Salt Lake was split into two halves:  the North and the South. Since the railroad company was in  the business of efficient travel and not studying waterway dynamics, they created a lake where the rivers pretty  much only flowed into the South half, leaving a stagnant North half.

Over time, salt levels built  up even more in the North, reaching 10 times higher than the average ocean. So today, pretty much the only things  that can live there are microorganisms called Haloarchaea that can  survive high salt concentrations. And the shift in lake dwellers was  pretty easy to see because haloarchaea make a purple colored protein that  turned the top half of the lake a totally different color than the bottom half!

In fact, the lake is now so salty  that the diatoms which told us so much about its past can barely survive. You can only find them living near the  freshwater rivers that flow into the lake. Even the southern half of the lake is still  four times saltier than the average ocean.

And that has some unappealing consequences. When organic material gets  trapped under dense salty water at the bottom of the lake, bacteria  that thrive without oxygen consume it and produce hydrogen sulphide  that smells like rotten eggs. Which stinks!

Because of the railway, the Great Salt Lake is essentially two separate ecosystems. To the point that a 2016 study comparing  the microbial communities in the lake found much greater diversity of species in  the less salty region south of the railway. Now it’s kind of fun superlative to be one  of the saltiest bodies of water on Earth, but what we see today is not  the sign of a healthy ecosystem for the plants and animals that  would have lived in Lake Bonneville.

What was once a giant freshwater habitat  has become one of the saltiest places on Earth, where only highly specialized  microscopic organisms can live. And, in a way, that brings us full circle. There’s a fundamental principle in  ecology that the health of an ecosystem can be measured through the diversity  and abundance of the species within it.

Essentially the more diverse an ecosystem  is, the healthier it tends to be. This is known as the Patrick  principle … named after, you guessed it, the same Ruth Patrick  who first studied diatoms in Utah. The Great Salt Lake has seen some  huge changes and continues to be a superlative in several ways.

But we wouldn’t know about all of that big stuff without the help of our  tiniest friends: the diatoms. So don’t forget to say thanks  as you accidentally swallow a mouthful of them the next time you  go swimming in the lake or ocean. [♪ OUTRO]