The idea that the Mediterranean had once dried up was first proposed more than two decades ago, when drill ships began pulling up extremely salty sediment cores from the seafloor. Salt deposits are left when seawater evaporates, and these dated from 7 to 5 million years ago, a time when much of the world’s water was locked up in polar ice and tectonic activity closed the Strait of Gibraltar, cutting off the sea from the Atlantic. Last May, geochemist Klaus Wallmann of the geomar Research Institute in Kiel, Germany, reported new evidence that those conditions had actually converted the Mediterranean into a deep, dry basin.
Off the coast of Crete, in a two-mile-deep depression called the Discovery Basin (indicated by yellow dot), Wallmann and his colleagues found a large underwater lake of extremely salty brine sitting above a thick deposit of salt. This is not your average brine: it is composed almost entirely of concentrated magnesium chloride, a salt that can only be produced when an evaporite salt called bischofite is redissolved in water. Bischofite is the very last salt that is formed when seawater evaporates, Wallmann observes. While we can’t say for sure that this means there wasn’t any water left in the Mediterranean, we know that more than 99 percent of it must have evaporated in order to produce the bischofite.