L'Atalante is only one of several such lakes underneath the normal waters of the Mediterranean. When the Strait of Gibraltar was closed, about six million years ago, the Mediterranean dried up and became a huge salt lake, rather like the Dead Sea and the Great Salt Lake are today. Then it dried out completely, leaving an enormous amount of salt. When the Straight of Gibraltar opened again about five million years ago, water from the Atlantic Ocean rushed in to form the Mediterranean we see today. Even today, the Atlantic feeds the Mediterranean with water, because the evaporation from the 'Med' is greater than is provided by its rivers. With its huge supply of salt underneath, and the constant evaporation, the Med is a high salt sea compared with the Atlantic.