The serene turquoise waters of the Mediterranean Sea hide a sharp-tasting secret: a layer of salt up to two miles thick, lurking deep underneath the basin. The ghostly white minerals are one of the few traces of an ancient Mediterranean Sea that vanished millions of years ago. Some scientists believe that the entire sea evaporated for a time, desiccated like the Sahara to the south.

Even after decades of study, the details surrounding the sea’s vanishing act and the torrents of water that refilled the basin remain an enduring mystery. The refilling of the Mediterranean about five million years ago may have been the biggest flood in our planet’s history. By one estimate, the cascade of water that filled the cavernous