At the western edge of the Mediterranean basin, a narrow land bridge once sealed off the Atlantic. We now call it the Gibraltar strait but 5.33 million years ago, it used to be a land bridge connecting Europe and Africa. At the time, the Mediterranean Sea was almost a salt flat, just a shadow of its current self. Then, the sea finally overtopped. What followed was a cataclysm of almost unimaginable scale—a flood that tore across continents and refilled an entire sea basin in as little as two years.

A new study provides the clearest evidence yet of the Zanclean megaflood, a geological event so immense it may have unleashed more water than any flood in Earth’s history. Moving at speeds of up to 72 miles per hour, and pouring in at rates up to 100 million cubic meters per second, the flood surged across the Mediterranean, funneled through canyons, and rushed into the desiccated void of the Mediterranean.

Now, researchers have stitched together the flood’s signature and confirmed that one of the planet’s greatest rehydrations was indeed a roar of planetary scale.

Cut off from the ocean

Our story begins during the Messinian Salinity Crisis, a dramatic episode that unfolded between 5.97 and 5.33 million years ago. The Mediterranean Sea fell victim to tectonic movements. As tectonic plates moved around, they sealed off the Strait of Gibraltar, essentially isolating the Mediterranean Sea. Gradually, the sea became a desert basin with kilometer-thick salt deposits.

The water vanished, leaving behind a geological wound visible from space.

But the Mediterranean is full of water now, so it must have filled back up. We now know that this happened some 5.33 million years ago, through the strait of Gibraltar. But how exactly did it happen? Was the sea refilled slowly, like a bathtub over millennia? Or did it explode back to life in a single, furious event?

The Mediterranean Sea is about two-thirds as big as the US, or about as big as five Spains put together. It’s so big that it seems hard to believe it could have filled back quickly; and yet, new research led by Aaron Micallef of the Monterey Bay Aquarium Research Institute leaves little room for doubt.

“The Zanclean megaflood was an awe-inspiring natural phenomenon, with discharge rates and flow velocities dwarfing any other known floods in Earth’s history,” Micallef said. “Our research provides the most compelling evidence yet of this extraordinary event.”

The first evidence comes from over 300 asymmetrical ridges found in a wide corridor stretching across the Sicily Sill—a submerged barrier that once separated the eastern and western Mediterranean basins. Their shape wasn’t random. These ridges had a streamlined geometry, aligned northeast, as if molded by an immense, rushing flow of water.

Beneath their tops, researchers found a jumble of rocks—poorly sorted breccia, made of pebbles and boulders from across the region. This is a signature of high-energy, short-lived flooding.

“The morphology of these ridges is compatible with erosion by large-scale, turbulent water flow with a predominantly north easterly direction,” says Professor Paul Carling, an Emeritus professor in the School of Geography and Environmental Science at the University of Southampton and a coauthor of the study.

But the evidence doesn’t stop there.

Signs of flood below ground

This wasn’t the first clue hinting at a massive flood, but the evidence was rather scattered. Researchers were lacking a smoking gun, and that smoking gun was probably underground. Such a flood would leave behind ancient channels and sediments, but these would be covered by now. To find stronger evidence of the megaflood’s path, you needed to look beneath the surface, and that’s what the study authors did.

They used a technique called seismic imaging, which works a bit like an ultrasound for the Earth. Scientists send sound waves into the ground using a specialized device and then record how these waves bounce back. These sound waves (technically, acoustic waves) are essentially the same as seismic waves. They are reflected by different rock layers in different ways, depending on their density and composition. By measuring the time it takes for the echoes to return and analyzing their strength, researchers can create detailed cross-sectional images of the subsurface.

With this method, they found an offshore 20-kilometer-wide, W-shaped channel carved into the shelf. It’s essentially an underwater valley in the eastern Mediterranean. This fits perfectly with the idea of a massive, sudden flood.

The shape and location of the channel suggest that it acted like a massive funnel. When the water surged in, it fed the waters towards the deeper parts of the Mediterranean basin. Researchers also created models to simulate this process, and these models once again fit with the observed data.

“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,” Micallef added. “It opens the door to further research along the Mediterranean margins.”

How big was the flood?

If you’d be gliding magically in the air as the Zanclean megaflood unfolded, you’d probably be stunned. This would have been a planetary-scale event, initially concentrated along one single strait.

At peak flow, water speeds may have reached 32 meters per second—about 72 miles per hour. The discharge rate may have topped 100 million cubic meters per second, making it the largest flood known on Earth and probably the largest in the solar system.

The Mediterranean Sea filled up in as little as 2 years and as long as up to 16 years. As crazy as that sounds, it’s the most likely theory with available data.

Geologists had suspected this flood for years. Now, they have the onshore-to-offshore trail to prove it—from ridge to canyon, from breccia to basin.

Like the asteroid that wiped out the dinosaurs, or the volcanic blasts that shaped early Earth, the Zanclean megaflood stands as a monument to how quickly the planet can change.

“Our findings demonstrate that the Messinian salinity crisis was terminated through a cataclysmic flood, the researchers conclude.”

The study was published in Nature Communications Earth & Environment.