Salts come out of rocks, often thanks to the rain. Rain is slightly more acidic than pure water, because carbon dioxide from the air combines with water to form carbonic acid. In most cases, rain is not acidic enough to harm plants or animals — although certain pollutants from factories and cars can make what’s called acid rain, which can cause all sorts of problems. But rain does cause rocks to erode, or slowly break down over time. Minerals, including various salts, break free from rocks and seep into the ground or water around them.
Rivers don’t get very salty, because minerals come out of rocks in small quantities. But rivers are constantly feeding water into the ocean, and that water doesn’t really leave once it arrives — unless it evaporates under the sun. And evaporation doesn’t get rid of the salt. Salt leaves the ocean only when living things use it or carry it out on their bodies.
When the planet was new, our oceans probably weren’t that salty. Their saltiness — called salinity — built up over time. The National Oceanic and Atmospheric Administration estimates that if you took all the salt out of the ocean and spread it out on land, it would form a layer 500 feet thick over the entire surface.
The ocean isn’t the only place that accumulates salty water. In fact, the Dead Sea — which is bordered by Jordan, Israel and the Palestinian territories, and considered one of the saltiest bodies of water on the planet — is actually a lake. Most lakes have water flowing out, not just in, which keeps salt from accumulating. But after water enters the Dead Sea from the Jordan River, it has no way to exit. And the lake’s location — in a very dry place, and sitting very far below sea level — means it evaporates quickly. The salinity is so high near the bottom of the Dead Sea that salt clumps together to form big crystals.
Although seawater is fun to play in, try your best not to swallow more than an accidental mouthful. Your body needs salt to survive, but you must urinate any extra amount before it poisons you. A glass of ocean doesn’t actually contain enough water to make up for the added salt. It won’t take too much ocean gulping before your body runs out of water. So make sure you stay hydrated while you enjoy the sea this summer!