Around 5.96 million years ago, during the late Miocene epoch, Earth witnessed one of the most dramatic geological events in its history: the Messinian Salinity Crisis (MSC). Due to tectonic shifts between the African and Eurasian plates, the connections between the Atlantic Ocean and the Mediterranean Sea (in the region of modern-day Gibraltar) closed.
Because the Mediterranean region experiences higher evaporation rates than it receives in precipitation and river runoff, the sea began to dry up. Over roughly 600,000 years, the Mediterranean Basin transformed into a scorching, mile-deep salt desert.
The idea that an entire sea could vanish seemed like science fiction until undeniable geological evidence was uncovered, primarily starting in the 1970s. Here is a detailed breakdown of the geological evidence that proves the Messinian Salinity Crisis occurred.
1. The Discovery of Massive Evaporite Deposits
The "smoking gun" for the MSC was discovered in 1970 during Leg 13 of the Deep Sea Drilling Project, conducted by the research vessel Glomar Challenger. Scientists, led by Kenneth Hsü and William B.F. Ryan, drilled deep into the floor of the Mediterranean.
Instead of finding continuous layers of typical oceanic ooze, the drill brought up cores containing evaporites—minerals that only form when salty water evaporates. * Gypsum and Anhydrite: The drills recovered these sulfate minerals, which precipitate out of seawater only when it is concentrated by high levels of evaporation. * Halite (Rock Salt): Beneath the gypsum, they found massive deposits of pure rock salt. * Stromatolites: The cores also contained fossilized mats of algae (stromatolites) that only grow in extremely shallow, sunlit, highly saline water—similar to the modern-day Persian Gulf. Finding these thousands of feet below the current sea level proved the water had evaporated away.
The sheer volume of these salt deposits is staggering. In some areas of the basin, the salt layers are up to 1.5 miles (2.5 kilometers) thick. A single evaporation of the Mediterranean could only leave a salt layer about 60 feet thick. Therefore, the immense thickness indicates that the basin experienced repeated cycles of partial refilling from the Atlantic followed by complete evaporation.
2. Seismic Mapping and the "M-Reflector"
Before the Glomar Challenger even drilled, geophysicists surveying the Mediterranean seafloor using seismic reflection (bouncing sound waves off the ocean floor to map subterranean rock layers) noticed a strange anomaly.
Beneath the soft, modern marine sediments, their sound waves hit a very hard, continuous layer of rock that reflected the seismic waves back with intense clarity. They named this anomalous layer the "M-Reflector" (M for Messinian).
Once the drilling confirmed that the M-Reflector was a massive cap of evaporite rock (gypsum and salt), seismic maps allowed scientists to trace it. They found that the M-Reflector blankets almost the entire Mediterranean basin, proving that the drying event was not localized but affected the entire sea.
3. Buried "Grand Canyons" of River Systems
One of the most fascinating pieces of evidence comes not from the sea, but from the rivers that feed it, such as the Nile and the Rhône.
Rivers erode the land down to what geologists call "base level"—which is usually sea level. When the Mediterranean evaporated, its surface dropped by as much as 10,000 feet (3,000 meters). Suddenly, the rivers flowing into the basin were miles above their new base level.
To reach the bottom of the dry basin, these rivers began cutting violently into the continental rock, carving out colossal gorges much deeper than the Grand Canyon. * The Eonile Canyon: During the construction of the Aswan High Dam in Egypt, engineers drilling into the bedrock under the modern Nile discovered a massive gorge buried under hundreds of feet of sediment. At Cairo, this buried canyon is over 8,000 feet deep. * The Rhône Gorge: Similar seismic surveys in France revealed that the Rhône River carved a deep canyon that extends far inland from the modern coast, completely filled in with sediment deposited after the sea returned.
4. Paleontological (Fossil) Evidence
The fossils found in the sediment cores provide a clear timeline of the ecological catastrophe. * Pre-Crisis: Deep-sea muds older than 5.9 million years contain abundant fossils of normal, deep-water marine organisms. * During the Crisis: In the evaporite layers, normal marine fossils completely vanish. They are replaced by species of ostracods and diatoms that can only survive in shallow, hypersaline "brine pools" or coastal lagoons. The cores also contain wind-blown dust and terrestrial plant spores, indicating the seafloor was exposed to dry, desert winds. * Post-Crisis: Immediately above the salt layers, the sediment abruptly shifts back to oceanic mud teeming with deep-sea marine fossils.
The Resolution: The Zanclean Flood
The abrupt return of deep-sea marine fossils perfectly marks the end of the Messinian Salinity Crisis about 5.33 million years ago. Geologic faulting and rising global sea levels caused the Atlantic Ocean to breach the sill at the Strait of Gibraltar.
This resulted in the Zanclean Flood. Water from the Atlantic poured into the dry basin. Geological estimates suggest the inflow was catastrophic—discharging water at a rate 1,000 times greater than the modern Amazon River. The immense, mile-deep basin of the Mediterranean may have refilled in just a few months to two years, ending the reign of the great salt desert forever.