Why is the Dead Sea so salty?

The Dead Sea is salty because it is a dead end. The Jordan River and a handful of streams flow in, but no river flows out. The only way for water to leave is to evaporate into the hot desert air, and evaporation takes only the water. Every mineral that the rivers washed out of the surrounding rocks stays behind, year after year. The result is water about 34% salt, 9.6 times saltier than the ocean.

The place is built for this. The lake's surface sits about 440 metres below sea level, the lowest land on Earth. Summer averages reach 32 to 39 °C, and the Judaean Mountains cast a rain shadow over it, so barely 50 to 100 millimetres of rain fall each year. Water arrives slowly and leaves quickly, and the salt keeps piling up.

That brine is so dense that anyone can lie back and float like a cork, which is why the Dead Sea was one of the world's first health resorts. It is also why it got its name: no fish and no water plants can survive in it, only tiny microbes.

The dead end is now shrinking. Since the 1960s, much of the Jordan's water has been diverted for people and farms, and the lake's surface has fallen from 1,050 square kilometres in 1930 to about 605 today.