Around5.6 million years ago, the Mediterranean Sea almost vanished. Cut off from the Atlantic Ocean, it gradually dried out, leaving behind a huge basin covered in salt and carved by rivers. Hundreds of thousands of years later, Atlantic waters returned and refilled the basin in what may have been one of the largest floods Earth has ever seen.
Geologists have been studying this episode for decades. While the main evidence is well established, researchers are still debating exactly how dry the Mediterranean became and how quickly it filled up again.
Today, the Mediterranean is one of the world’s most recognizable seas, bordered by Europe, Africa and the Middle East. Yet its history includes a period when much of the basin looked more like a vast desert than an inland sea. The event is known as the Messinian Salinity Crisis, and it has left traces that can still be found beneath the modern seafloor.
A Giant Salt Basin Hidden Beneath The Sea
The first major clues emerged in 1970 when the research vessel Glomar Challenger drilled into the Mediterranean seafloor. What researchers found was unexpected: thick layers of salt and gypsum buried beneath younger sediments.
According to an article published by Nature, the Mediterranean accumulated more than one million cubic kilometres of salt during this period. That makes it one of the largest known salt deposits on the planet. For geologists, these deposits are difficult to explain without large-scale evaporation. As water evaporates, salt becomes increasingly concentrated until it begins to crystallize and settle on the basin floor.
The drilling results were backed up by other discoveries. Scientists also foundfossil soils, remains of land plants and evidence of ancient river systems. These findings pointed to a Mediterranean that was very different from the sea we know today.
Rivers Shaped A Sea That Was No Longer There
For roughly 600,000 years, much of the Mediterranean basin was dramatically reduced. Instead of open water stretching between countries, there were salt flats, isolated lakes and deep valleys carved by rivers. The Nile cut a canyon more than a kilometer deep beneath its present-day course in Egypt. Similar features were created by the Rhône in France and the Po in Italy.
Late in the crisis, a stage known as the Lago Mare phase saw shallow brackish lakes spread across parts of the basin. Their animal life resembled species living in the modern Caspian Sea.
Conditions at the bottom of the basin were likely extreme. Some areas lay several kilometres below sea level, creating higher air pressure and very high temperatures. Researchers have suggested that parts of the former seafloor may have reached temperatures comparable to some of the hottest places on Earth today.
A Flood That Is Still Sparking Debate
The Mediterranean eventually returned, but exactly how it happened remains debated. A 2009 study published in Nature by Daniel Garcia-Castellanos and colleagues argued that the reopening of the Strait of Gibraltar triggered the Zanclean Flood, rapidly refilling the basin. The researchers estimated that around 90% of the Mediterranean may have filled within months to two years.
More recent findings support this scenario. A study, published in Communications Earth & Environment, reported that a team of researchers identified more than 300 erosional ridges and a 20-kilometre-wide channel in southeastern Sicily, interpreted as traces of powerful floodwaters.
“These findings not only shed light on a critical moment in Earth’s geological history but also demonstrate the persistence of landforms over five million years,” said Aaron Micallef, who led the study. “It opens the door to further research along the Mediterranean margins.”
At the same time, the catastrophic flood scenario is being re-examined. Based on a February 2026 review published by Knowable Magazine, several recent studies suggest the Mediterranean may never have become completely dry and that some connection with the Atlantic could have remained in place during parts of the crisis. Other researchers argue that the refilling process may have taken thousands of years rather than months.
