Context: Around the 1920s, German architect Herman Sörgel came up with an idea to drain water from the Mediterranean sea by building a dam across the strait of Gibraltar, among other places. The goal was to reduce the sea level enough to claim more land from the sea and bring Europe closer to Africa so that Europe could colonize it and hedge against the growing power of the US post WW1. However, with the fall of colonialism and invention of nuclear power after WW2, the idea lost support.
There used to be an ocean above todays central Europe which completely evaporated. That's where all the underground salt comes from.
https://en.wikipedia.org/wiki/Germanic_Basin