The Great Drying
Between 5.96 and 5.33 million years ago, the Mediterranean Sea experienced a large geological event known as the Messinian Salinity Crisis. Tectonic plate movements between Africa and Eurasia gradually closed the ancient seaways that connected the Atlantic Ocean to the Mediterranean. Cut off from its primary source of water, the sea began to evaporate much faster than rivers could replenish it.
This isolation led to a dramatic drop in sea level, with the basin floor sinking between 1.5 and 2.5 kilometers below the global ocean level. As the water vanished, the remaining sea became a series of hypersaline lakes. The extreme concentration of dissolved minerals caused them to precipitate out of the brine, blanketing the seabed. This process laid down a colossal layer of evaporite minerals, primarily gypsum, anhydrite, and halite (common salt). In some areas, these salt deposits reach a thickness of over 3 kilometers, with a total estimated volume exceeding 1 million cubic kilometers. This event also carved the landscape, as rivers like the Nile and Rhône cut deep canyons into the newly exposed continental margins, with the Nile's canyon reaching 2,500 meters below sea level near modern-day Cairo.
The Zanclean Deluge
The crisis ended abruptly 5.33 million years ago with one of the most catastrophic floods in Earth's history. The natural dam at the Strait of Gibraltar breached, allowing the Atlantic Ocean to pour back into the desiccated basin. This event, known as the Zanclean Flood, was of massive proportions.
The flow rate is estimated to have reached 100 million cubic meters per second, a volume about 1,000 times greater than the modern Amazon River. Water cascaded down a drop of more than a kilometer, creating what might have been the largest waterfall ever to exist on the planet. At the flood's peak, the sea level inside the basin may have risen by more than 10 meters per day. The entire Mediterranean basin, which had taken hundreds of thousands of years to dry up, was likely refilled in a period as short as a few months to two years. The force of the water carved a massive channel through the Strait of Gibraltar, the remnants of which are still visible in modern seafloor data. This sudden influx of water also triggered earthquakes as the immense weight pressed down on the Earth's crust.