Here's a puzzle you've swum in your whole life without noticing. Rain is fresh. Rivers are fresh. Every drop of water that flows into the sea started out drinkable. And yet the sea it all pours into is salty enough to sting your eyes and ruin your thirst. Where does the salt come from, if everything feeding the ocean is fresh? The answer is a slow, patient robbery of the land, and a trap the salt checks into and can never leave.
01 · The thiefRain eats rock
It starts in the sky, with rain that isn’t quite as pure as it looks. As raindrops form and fall, they absorb a little carbon dioxide from the air and turn faintly acidic. That mild acid is enough to slowly dissolve the rock and soil it lands on, prising loose tiny amounts of minerals, including sodium and chloride, the two halves of ordinary salt. No single rain shower does much. But rain has been falling on rock for billions of years, and drop by drop it’s been quietly chewing the continents apart and washing the pieces downhill.
02 · The deliveryRivers carry it to the sea
Where does that dissolved rock go? Into streams, into rivers, and out to the ocean. Each river carries only a whisper of salt, far too little to taste, which is exactly why rivers seem fresh. But add it all up and the world’s rivers deliver something like four billion tons of dissolved salts to the sea every single year. The river isn’t salty; it’s a pipeline, forever moving a thin trickle of salt from the land to one enormous collecting basin. And in that basin, something crucial happens that never happens in the river.
03 · The trapWater leaves, salt can't
This is the heart of it. The sun beats down on the ocean and lifts water off its surface as vapour, which rises, forms clouds, and falls again as fresh rain. But here’s the trick: water can evaporate, salt cannot. When the sea gives up water to the sky, every grain of dissolved salt is left behind. The fresh rain then falls on land, dissolves a little more rock, and runs back to the sea carrying its new load. Every turn of this cycle brings salt in and takes only pure water out. The ocean is a one-way trap: easy for water to leave, impossible for salt.
04 · The recipeWhy it's the same as table salt
So what is sea salt, exactly? Reassuringly ordinary. About 85% of the dissolved matter in seawater is sodium and chloride, the two ingredients of the salt in your kitchen. The rest is a supporting cast of magnesium, sulphate, calcium and potassium. There’s a neat reason sodium and chloride dominate: sea life barely uses them. Other elements that wash into the ocean get eagerly grabbed by living things and pulled out of the water, but sodium and chloride are left largely untouched, so they’re the ones that pile up. The sea is salty, in the end, because salt is the leftover nobody wanted.
If salt has been pouring in for billions of years, why isn't the sea getting saltier and saltier? Because it's also secretly draining out. Sea creatures lock salt into shells, minerals settle to the sea floor, and dried-up shallow seas leave salt beds behind. Input and removal have balanced out, holding the sea at a steady saltiness for a very long time.
05 · The extremeWhat the Dead Sea shows
Want to watch this process run without brakes? Look at the Dead Sea. It’s a landlocked lake in a hot, dry valley: rivers flow in carrying salt, but crucially, no river flows out. The only exit for water is evaporation, and in that heat the water leaves fast, abandoning its salt with nowhere to go. So the salt concentrates, and concentrates, until the Dead Sea is around ten times saltier than the ocean, dense enough to float a reader like a cork. It’s the same salt-trapping mechanism as the whole ocean, just sealed shut and left to bake.
06 · The payoffSo why is the ocean salty?
Because for billions of years, rain has been dissolving the land and rivers have been carrying the salt to one great basin that lets water out but keeps the salt in. Add a steady seep of minerals from volcanic vents on the sea floor, subtract the salt that life and sediment quietly bury, and you get an ocean holding steady at about three and a half percent salt, most of it the very same sodium chloride on your dinner table. The rivers really are fresh. The sea is just the place where all their salt has been going, and staying, since almost the beginning of the world.
Quick questions
Why is the ocean salty in simple terms?
Rain slowly wears away rock on land and dissolves tiny amounts of salt out of it. Rivers carry that salt to the sea. Then the sea loses water to evaporation, but the salt can't evaporate, so it's left behind. Do that for billions of years and the salt keeps building up, which is why the ocean is salty even though the rivers feeding it are fresh.
Where does the salt in the ocean actually come from?
Two main places. The bigger source is the land: slightly acidic rainwater erodes rocks and washes dissolved minerals, including sodium and chloride, down rivers into the sea. The second source is the sea floor itself, where hydrothermal vents and undersea volcanoes release dissolved minerals straight into the water. Together these have supplied the ocean's salt over geological time.
If rivers carry salt to the sea, why isn't river water salty?
Because river water is salt on the move, in tiny concentrations, on a one-way trip. A river carries a very small amount of dissolved salt, far too little to taste, and delivers it to the ocean. There, the salt accumulates because water keeps leaving by evaporation while the salt stays put. The river is the delivery pipe; the ocean is the collecting basin where the salt piles up.
Why does the salt stay in the ocean instead of leaving?
Because of a simple trap: water can evaporate, salt cannot. Sunlight lifts pure water vapour off the ocean's surface into the sky, leaving every dissolved salt behind. That vapour falls as fresh rain, runs off the land gathering a little more salt, and returns to the sea. Each turn of the cycle adds salt and removes only water, so the ocean acts as a one-way collector.
What is sea salt actually made of?
Mostly the same stuff as table salt. Around 85% of the dissolved material in seawater is sodium and chloride, the two ingredients of common salt. The rest is a mix of other ions such as magnesium, sulphate, calcium and potassium. So the sea is not just salty in a vague sense; it's genuinely rich in ordinary sodium chloride, plus a supporting cast of other dissolved minerals.
How salty is the ocean?
On average, about 3.5% salt by weight, often written as 35 parts per thousand. That means every kilogram of seawater holds roughly 35 grams of dissolved salts. It varies by place: seas with lots of evaporation and little fresh input, like the Mediterranean or the Red Sea, are saltier, while areas near big river mouths or melting ice are fresher. The Dead Sea is an extreme, many times saltier still.
Why is the sea getting saltier, or is it staying the same?
Overall the ocean's saltiness has been roughly stable for a very long time, because salt is also removed as fast as it arrives. Salts don't just build up forever: they get locked away when sea creatures use them for shells, when minerals settle onto the sea floor, and when shallow seas dry out and leave salt deposits. Input from rivers and vents is balanced by this slow removal, keeping the average steady.
Why is the Dead Sea so much saltier than the ocean?
Because it's a trap with no exit. The Dead Sea is a landlocked lake in a hot, dry region: rivers flow in carrying salt, but no river flows out, so the only way water leaves is by evaporation. Water evaporates fast in the heat and the salt is left behind with nowhere to go, concentrating to around ten times the saltiness of the ocean. It's the ocean's salt-trapping process cranked to an extreme.
Could you drink seawater in an emergency?
No, and it's dangerous to try. Seawater is so salty that drinking it forces your body to use more water than it gains, drawing water out of your cells to flush the excess salt through your kidneys, which worsens dehydration. That's why castaways surrounded by ocean can still die of thirst. The very saltiness this question is about is exactly what makes the sea undrinkable.
Has the ocean always been salty?
The oceans have been salty for most of Earth's history, since very early after they first formed billions of years ago. Salt began accumulating as soon as there was rain, rock and rivers to carry dissolved minerals to the sea. The exact saltiness has drifted over geological time, but there was never an era of large fresh oceans that later turned salty. The sea has been salty for about as long as there has been a sea.
Why are most lakes fresh if they collect river water like the ocean?
The key difference is an exit. Most lakes have a river flowing out of them as well as in, so water and dissolved salt both keep moving through and onward, eventually heading toward the sea. Because the salt never gets trapped, it never builds up. The lakes that do turn salty, like the Dead Sea or the Great Salt Lake, are the ones with no outflow, where water can only leave by evaporating.
Can seawater be turned into drinking water?
Yes, through a process called desalination, which removes the salt by methods like forcing seawater through fine membranes or distilling it. Many dry coastal regions rely on desalination plants for fresh water. The drawback is that it takes a lot of energy and money, so it isn't a simple fix everywhere, but technically the salt that makes the sea undrinkable can be taken back out.
Why doesn't the sea freeze as easily as fresh water?
Because dissolved salt lowers the freezing point of water. Fresh water freezes at 0 degrees Celsius, but average seawater has to be chilled to about minus 1.9 degrees Celsius before it turns to ice. It's the same reason salt is scattered on icy roads. When sea ice does form, much of the salt is squeezed out, so the ice itself ends up far fresher than the water it came from.
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