Have you ever taken a sip of seawater and immediately regretted it? That salty sting on your tongue might’ve left you wondering—why is sea water so salty, while river water tastes fresh, even though seawater originally comes from rivers? It seems like a simple question, but the answer involves fascinating natural processes that have been happening for millions of years.

You may not believe it, but the ocean was not always salty. Hundreds of billion years ago, when Earth was still a hot, fiery ball floating in space, the first oceans were formed from volcanic steam and meteorites falling onto the planet. Back then, seawater had no salt at all—it was as pure as today’s rainwater. So where did the salt come from?

There are many causes as to why sea water became salty, one of them being the erosion of rocks over time. Rain and other forms of precipitation fall on our planet as slightly acidic fresh water (because it absorbs CO2 from the air, forming carbonic acid). As some of this water washes over the landscape, it physically erodes rocks and chemically breaks them down by dissolving minerals, including sodium and chlorine—the two key ingredients of salt and the flow of water in rivers transports these salts to the ocean. The ocean collects all the salt from rivers and when evaporation occurs, all the salt is left behind and it builds up over time. This cycle of mineral deposition and accumulation, having occurred for millennia, has concentrated the salts in the ocean so much that it is no longer fresh but saline. It is estimated that the rivers and streams flowing from the United States alone discharge 225 million tons of dissolved solids and 513 million tons of suspended sediment annually to the ocean.

Throughout the world, rivers carry an estimated four billion tons of dissolved salts to the ocean annually. On average, sea water contains about 3.5 percent salt. That salt makes sea water denser than freshwater. The increased density makes people, animals, and other objects more buoyant in seawater. And not all the oceans have the same salinity, the Dead Sea is about ten times saltier than normal seawater, while the Baltic Sea is much fresher because so much freshwater flows into it, which is why people can effortlessly float in the Dead Sea.

Since salt is transported from rivers to the ocean, why is river water not salty? Well, river water is not salt-free, there are some rivers in the United States and Australia that have a higher amount of salt than the others. The answer is the speed of waterflow in rivers. Rivers are constantly moving, carrying away any salt before it has time to build up like it does in the ocean. Imagine pouring a pinch of salt into a bowl of water while constantly changing the water—the salt gets washed away before it can make the water salty. That’s exactly how rivers work. However, there are a few salty rivers in hot, dry areas where water evaporates faster than it flows. In those places, salt can accumulate over time.

So, what if the ocean turned into freshwater? It sounds like a dream, allowing people to freely use ocean water to drink. But, that would be a global nightmare. First, marine ecosystems would be completely wiped out. Fish, coral, and all kinds of sea creatures have adapted to saltwater for millions of years. If the ocean suddenly became freshwater, their cells would swell and burst from osmotic shock. Next, the global climate would go wild. Ocean salinity helps regulate ocean currents—these “highways” transport heat around the planet. If salt disappeared, key currents like the Gulf Stream would shut down, plunging Europe into a new ice age, while other regions would face uncontrollable storms and super droughts. Salt actually helps control the growth of microorganisms. Without it, harmful algae would grow out of control, turning the ocean into a toxic green soup.

However, as rivers keep dumping salt into the ocean, will it one day become too salty to support lives? The ocean has an incredibly smart salt-balancing system. About the same tonnage of salt from ocean water received from the rivers  is deposited as sediment on the bottom of the ocean and thus, yearly gains mostly offset yearly losses. In other words, the ocean today probably has a balanced salt input and output. Many marine organisms also use salt to build their shells, which removes salt from the water. And geological activity beneath the seafloor helps recycle salt too—keeping ocean salinity stable for many years to come.