Why is the Dead Sea so salty?

The Dead Sea is a terminal lake, meaning water enters from the Jordan River and surrounding runoff but has no outlet to any ocean. The only way water leaves is evaporation, which removes pure water and leaves every dissolved mineral behind. Over thousands of years in an extremely hot, dry climate, that process has concentrated salts to roughly 34 percent, around ten times the salinity of seawater. The resulting density is high enough that a human body floats effortlessly at the surface, which is the reason for every photograph of someone reading a newspaper on their back.

The same geography makes it fragile. The lake sits at the lowest land elevation on Earth, more than 1,400 feet below sea level, in the Jordan Rift Valley. Since the 1960s, upstream diversion of the Jordan River for agriculture and drinking water, along with mineral extraction, has cut inflow drastically, and the surface has been dropping more than three feet per year. The shoreline has retreated substantially and thousands of sinkholes have opened as freshwater dissolves buried salt layers. The chemistry also differs from ocean salt, being dominated by magnesium and calcium chlorides rather than sodium chloride, which is why the water feels oily and why swallowing it is genuinely dangerous rather than merely unpleasant.