Why is the Ocean Water Salty?

Why is the ocean salty? Most people assume they know the answer — but the full truth goes much deeper than anyone expects.

Rain is fresh. Rivers are fresh. The water feeding the ocean has always been fresh. So where did all the salt come from? And why hasn't billions of years of fresh water flowing in washed it all away? The answer involves processes happening miles beneath the ocean floor — processes that science didn't even discover until 1977.