5.3 million years ago, the world’s largest flood refilled the Mediterranean

The Mediterranean Sea connects to the vastness of the Atlantic through the smallest of natural channels: The Strait of Gibraltar is only about 8 miles wide. People have swum across the Strait, and it is easy to see from Spain to Morocco on a normal, clear day. It seems like it would not take much for nature to simply close that connection between Europe and Africa, and in so doing isolate the Mediterranean Sea. Indeed, this has happened before.

About six million years ago, something caused the Mediterranean to be cut off. Perhaps it was an ice age that decreased the sea level enough to leave a land bridge between Spain and Morocco. The Mediterranean’s isolation could also have been the outcome of tectonic processes. 

In what is dubbed the Messinian salinity crisis, the Mediterranean Sea then evaporated over a period of about 1,000 years. The area turned into a dry basin, 3 to 5 km below sea level, punctuated with salty lakes similar to today’s Dead Sea. As slowly as the crisis set in, it ended with dramatic speed, in a megaflood the likes of which the world had never seen.

When the Mediterranean went dry

There is interesting evidence to show how comprehensively the Mediterranean ran dry. 

Massive rivers like the Nile and the Rhône today empty into the Mediterranean at sea level. But during the period in question, these rivers reached the bed of the Mediterranean. In the process, they carved out great canyons that arrive at the seabed, 2,700 meters below sea level. At Cairo, the Nile’s canyon was discovered during excavations for the Aswan Dam. Subsequent cores and surveys revealed a formation larger than the Grand Canyon. A similar canyon was found by the Rhône River, one that was carved out across a period of less than 100,000 years.   

Cores under the Mediterranean Sea itself revealed minerals such as gypsum and rock salt that are commonly deposited when salty water dries up. In some places, salt deposits were found up to 1 km thick.   

The drying of the Mediterranean also may have made its way into myth. The promontories rising on each side of the Strait of Gibraltar are known as the Pillars of Hercules. In 77 AD, Pliny the Elder described a legend according to which Hercules cut a path through these two rocks, joining the empty Mediterranean to the Atlantic Ocean once more. 

The Zanclean flood refills the Mediterranean

The Messinian salinity crisis came to an abrupt end in what was quite possibly the largest flood in the history of the planet about 5.3 million years ago. The land bridge between modern-day Spain and Morocco was breached, which opened the gates to the Zanclean flood. This megaflood carved a 200-km channel through the strait, perhaps deepening it about 40 cm per day. An enormous amount of water quickly refilled the Mediterranean. Most of the basin was refilled in no more than two years — and perhaps in as little time as a few months — with the water deepening about ten meters a day at its peak.   

Preserved underwater erosion features let us know exactly how strong the flood was. To see an example, we travel beneath the Alboran Sea, just to the east of the Strait of Gibraltar. Under the water lies an ancient volcano with an enormous amount of sediment deposited behind it — 35 km long, 7 km wide, and 163 meters thick. Although the evidence is not conclusive, scientists believe this deposit was likely left by a huge flood as it flowed around the volcano. Erosion features suggest that this rush of water was about 1,000 times stronger than the Amazon, meaning that roughly 100 million cubic meters of water rushed into the Mediterranean every second. 

A massive underwater canyon

But how extensive was this flood? If it filled the entire Mediterranean, it would leave evidence in other places as well. Running along the eastern edge of Sicily is an underwater cliff called the Malta Escarpment that is truly massive — 290 kilometers long and almost 4 kilometers high. As water tumbled over the Malta Escarpment, it is possible that it ran through the Noto Canyon. Noto, at 700 meters high, is steeper and deeper than any other canyon of its type in the world. Water might have flowed through this narrow entry into the eastern Mediterranean. 

On the eastern side of the escarpment near Noto Canyon is a deposit of sediment reaching up to 800 meters thick. The nature of this deposit is distinctive enough that scientists believe its origin is a great flood. It is a mixture of boulders, sand, and pebbles of all shapes and sizes — a mix similar to what you would expect floodwaters to deposit. If this is indeed from the Zanclean flood, this means that the waters would have reached all the way to the eastern Mediterranean, near Sicily, making it the largest megaflood deposit in the world.  

The Mediterranean is doomed

One day, the Mediterranean might dry up again. If the Strait of Gibraltar closes, the Mediterranean would likely run dry in about 1,000 years. Over a longer timeframe, the African plate is moving farther and farther to the north and east. Eventually, the heavier European continent will slip underneath it, and the Mediterranean will disappear — forever this time.