The largest known flood in geological time took anywhere from 2 years to 16 years to fill up the Mediterranean Sea. While it didn’t fill up the area fast, the water was powerful and hit some areas at speeds of 67 miles per hour — unleashing a wall of destruction that dug out canyons, shaped future islands and wiped out more than 95 percent of known marine species at the time.
“What makes this event extraordinary is it is the largest flood that we know that ever happened in Earth’s history,” says Aaron Micallef, a marine geoscientist at Monterey Bay Aquarium Research Institute, a nonprofit oceanography center. “It was perhaps the most dramatic even that our planet witnessed since 65 million years ago when the meteor hit Mexico’s Yucatan and led, among other things, to the demise of the dinosaurs.”
The Zanclean Megaflood
Scientists have argued some time over that the Mediterranean experienced a giant flood caused by an influx of water coming through the narrow Strait of Gibraltar. The strait, where Spain is only a few miles away from the coast of Morocco, was blocked up around 6 million years ago, isolating the Mediterranean from the rest of the Atlantic Ocean. The Mediterranean began to dry up and empty out, leaving two smaller basins in the east and west, sitting roughly on either side of Sicily and Malta.
“Basically, you could walk from Libya to Italy, or whatever was there at the time,” Micallef says.
But most of the Mediterranean remained below sea level, creating the recipe for a catastrophe. This occurred around 5.3 million years ago, when the Atlantic breached the Strait of Gibraltar once again, filling up the two basins to make what we know today as the Mediterranean Sea. The event is known as the Zanclean megaflood.
The trouble is, most of the previous evidence came from salt deposits — some kilometers thick — on the Mediterranean seafloor — an indication of what scientists call the Messinian salinity crisis. These were salt deposits that were left on the ground as the seawater dried up. But some questioned whether these salt traces could have been caused by other sources.
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Evidence of the Massive Flood
In 2018, Micallef and his colleagues published a study announcing the discovery of large sediment deposits to the southeast of Sicily and east of Malta similar to what was left by a massive flood. They also discovered an underwater canyon — one of the largest of the world — that was likely carved out by the tsunami-like flood.
“This channel is something you wouldn’t expect in such a shallow area,” he says.
In a more recent study published a few months ago in Communications Earth and Environment, Micallef and his colleagues went on land to find evidence of the Zanclean flood. The team descried up to 300 elongated ridges found across the entire region of southern Sicily.
“These ridges are mostly oriented in the same direction as you’d expect the flood to be,” Micallef says.
A lot of the outcrops had strange features, and were made up of gravel, pebbles, and even larger stones that would have required a powerful surge of water to transport them all to the area.
Filling the Mediterranean Sea
The refilling of the sea happened in spurts. Water was likely leaking through the Strait of Gibraltar for some time before it was fully breached, Micallef says. But when the flood came, it came fast — perhaps as much as 50 meters per second (67 miles per hour) in some parts.
“That would be almost like a tsunami,” Micallef says — it would have been larger and more destructive.
This water carried massive levels of deposit, the kind Micallef and his colleagues discovered southeast of Sicily, and carved underwater canyons with a large underwater channel off the Mediterranean coast of Morocco, he says.
There would have been no escaping its path, for both marine or land creatures. In fact, one study estimated that of 2,006 marine species known to have lived in the Mediterranean when it was isolated, only 86 survived extirpation.
“Only 4 percent of marine species made it,” Micallef says.
But while some of the flood happened quickly, the Mediterranean probably didn’t stabilize and fill up completely for anywhere from two to 16 years, Micallef said. Some days the sea level may have risen 32 feet, he says.
Read More: Early Humans Likely Used Dugout Canoes to Travel the Open Sea 8,500 Years Ago
Europe and North Africa Evolution
For Micallef, learning more about this flood can teach us how modern Europe and North Africa evolved, biologically and geologically. The flood likely affected the whole world, causing large-scale changes for the climate as well.
But also, learning about the destructive forces unleashed by such a flood can teach us more about what might happen when large glacial lakes burst due to climate change-driven ice melting.
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Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.
