Most of the salt in the ocean comes from a process that takes place on land. Rainfall contains carbon dioxide from the atmosphere, making it acidic. As rain erodes rocks on land, it releases ions – atomic particles that carry an electric charge. Rivers and streams carry these dissolved ions out to the ocean.
Some are removed from the water by various plants and animals, while other ions – mainly sodium and chloride – remain and become more concentrated. These two ions are what make seawater salty. It’s estimated that if all of the oceans were evaporated and their salts were spread evenly on the entire surface of the Earth, it would form a 152-metre (500-foot)-thick layer.