About 5.33 million years ago, according to the leading theory, the Atlantic broke through the land at Gibraltar and poured into a Mediterranean that had shrunk to brackish lakes. This is the Zanclean flood, named after the first age of the Pliocene. It ended the Messinian salinity crisis and made the Strait of Gibraltar. If the models are right it was the largest known flood on Earth. Nobody can say exactly how large, because that depends on how low the sea stood when the water arrived, and some geologists argue it never happened at all.
Months, by one group’s reckoning
The figure most often repeated comes from one group’s modelling. In 2009 García-Castellanos and colleagues worked out how water crossing the barrier would erode its own channel, letting more water in, which would cut faster still. In their model a low inflow, perhaps lasting thousands of years, gave way to a flood that filled about 90 per cent of the basin in between several months and two years. Peak flow reached about 100 million cubic metres per second at more than 40 m per second, and at times the level in the basin rose by over 10 m a day.
A 2024 paper by Micallef’s team quotes earlier estimates of 68 to 100 million cubic metres per second lasting 2 to 16 years, and the older view allowed up to 10 000 years. How much water was missing is the unresolved question, and a higher level would mean a much smaller flood. As it filled, the level of the world’s oceans dropped by about 10 m.
A ramp at Gibraltar, a cliff off Sicily
Popular images show a towering waterfall at Gibraltar, but the buried channel descends gradually, so the water seems to have run down a long ramp, a drop of more than 1 000 m. That channel runs nearly 390 km from the Gulf of Cádiz to the Algerian Basin. Its cutting removed about 1 000 km³ of rock, much of it left as giant bars in the Alboran Sea.
The western basin filled first, because the sill between Sicily and Africa held the water back. When it overtopped that barrier, it fell into the eastern basin down the Malta Escarpment, a limestone cliff over 3 km high in places. Micallef’s team proposes a fall of about 1 500 m there, through the Noto Canyon, with water moving at up to 160 km per hour.
What it left on the seabed and on land
Below the escarpment, in the Ionian Sea, lies a buried wedge of chaotic sediment 760 to 860 m thick at its thickest and 1 430 to 1 620 km³ in volume. Micallef’s team, who described it in 2018, called it the largest known megaflood deposit on Earth. It is a jumble of angular pebbles and boulders with no layering.
In 2024 the same team reported more than 300 streamlined ridges on the sill in south-east Sicily, on average 640 m long and 40 m high, all aligned with the flow and capped with rubble lying between Messinian and early Zanclean rocks. A shelf channel 8 to 20 km wide links them to the Noto Canyon. The authors conclude that the crisis ended in a cataclysmic flood, which would mean the sea had fallen a long way beforehand.
The case against
The flood remains a hypothesis. A 2025 review by Roveri, Lugli and Manzi argues that the connection through Gibraltar was probably active before and throughout the crisis, which would allow slow processes to have shaped the strait and a Mediterranean that never emptied. Geology along the southern edge of the Alboran Sea has been read as evidence against a catastrophe. Others propose a gradual refill, partly from other sources, or several pulses with a smaller flood first.
The sea that came back
If the flood happened as modelled, it cut the land route between Africa and Europe, finally isolated islands such as Crete, and let whales and other sea animals move in from the Atlantic. Quakes set off by the rapid filling may have caused landslides and tsunamis up to 100 m high. The Nile’s canyon became a long inlet. Older accounts take it as far as Aswan, about 1 200 km upstream, but Gvirtzman’s team found deep water only as far as the Cairo region, with a shallower arm beyond. Pliny the Elder recorded a much older explanation for the strait: Hercules dug it.
