Why is the Mediterranean sea water so salty?

Answer 1

The evaporation of the Mediterranean sea exceeds the amount of water it receives from all the rivers running into it. The missing water is replaced by, cold and above typical salt concentration, water from the Atlantic flowing in from the bottom of the Strait of Gibraltar.

During WWII German subs would enter the Mediterranean by going in by riding the incoming flow near the bottom of the strait and leave by going just under the surface.