Today tourists flock to the Mediterranean Sea for its beaches and shimmering blue-green waters. It wasn’t so hospitable some 6 million years ago, when the entire sea lost its Atlantic Ocean connection and temporarily dried up, leaving behind a kilometer-thick layer of salt. The basin’s original biodiversity was decimated. “Marine life after this crisis was never the same,” says Konstantina Agiadi, a geologist at the University of Vienna.

Geologists drilling into the bottom of the Mediterranean first discovered the salt layer, known as a “salt giant,” in the 1970s. Researchers eventually concluded it formed some 6 million years ago as continental shifts gradually severed the Mediterranean’s only major connection to the Atlantic. Over 600,000 years, water evaporated, depositing gypsum and salt. Just a few isolated briny ponds were left in the landlocked basin. The sea had teemed with life, including extensive coral reefs and warm-water fishes, but the basin now grew almost uninhabitable.