There is a specific version of the story that has been told many times.
Six million years ago, the Mediterranean Sea was disconnected from the Atlantic Ocean. Cut off from its water supply, it evaporated — over hundreds of thousands of years — into a series of hypersaline lakes at the bottom of a basin three to five kilometres deep. Where blue water separates Europe from Africa today, there was salt flat, exposed rock, and the occasional briny lake, all baking under the Mediterranean sun.
Then, about 5.33 million years ago, the Atlantic broke through at Gibraltar. Water spilled into the empty basin. It carved a channel through the rock. The channel deepened. More water rushed through. Within somewhere between a few months and two years, according to one prominent model, the entire Mediterranean refilled — with peak sea level rise possibly exceeding ten metres per day.
This story has been the standard geological picture for fifty years. As of the last few years, geologists are no longer sure it is right.
What the evidence definitely shows
Some parts of the story are not in dispute.
Around 5.96 million years ago, the connection between the Mediterranean and Atlantic became severely restricted. Salt deposits recovered from drilling projects across the Mediterranean seafloor show that during the following 600,000 years, an extraordinary mass of salt — approximately one million cubic kilometres, or roughly 6 percent of all the salt currently dissolved in the world’s oceans — precipitated out of the sea as its water evaporated. This is the specific geological event called the Messinian salinity crisis.
The evidence for extreme sea level drop is unambiguous. Rivers that emptied into the Mediterranean during this period cut deep canyons downward toward the retreating shoreline. The Nile carved a channel more than 2,500 metres deep beneath what is now Cairo. The Rhône cut a similarly deep gorge beneath what is now Marseille. These canyons were later buried by sediment when the water returned, but their profiles remain visible in seismic imaging. Rivers do not cut kilometre-deep canyons unless their outlet has fallen dramatically.
So the Mediterranean did lose a very large amount of water. And it did refill, at some point, to something resembling its modern volume. Those two facts are not seriously contested.
What is contested is everything in between.
Where the megaflood model came from
The specific catastrophic refill scenario — the one with sea level rising ten metres per day — was proposed in a 2009 paper in Nature by Daniel García-Castellanos and colleagues at the Spanish research council CSIC.
The team studied a specific 200-kilometre-long channel that runs east-west across the Gibraltar Strait, cut more than 250 metres deep into the underlying rock. Borehole and seismic data confirmed the channel’s continuity. The question was what had cut it.
García-Castellanos and his team applied an erosion model originally developed for mountain rivers, and ran it against the specific dimensions of the Gibraltar channel. Their model showed that a self-amplifying flood could account for the observed geometry. As water spilled over the initial barrier and began cutting downward, the deepening channel would allow more water through, which would erode more rapidly, which would allow still more water, and so on. Their calculations suggested that 90 percent of the total water transfer happened in a period of somewhere between a few months and two years, with peak flow rates of approximately one thousand times the current discharge of the Amazon River.
The paper was a scientific and popular sensation. It came with striking imagery — a three-kilometre-high waterfall, a wall of water rushing across an exposed seabed — that gave the geological event a specific visual shape. David Attenborough narrated a documentary version. Gibraltar issued a commemorative postage stamp. The megaflood entered public geological consciousness as the largest single flood in Earth’s history.
What the current debate is actually about
Over the past several years, a growing number of geologists have argued that the catastrophic refill model may be wrong in significant ways.
The most direct challenges have come from researchers including Guillermo Booth Rea at the University of Granada and colleagues studying the Mediterranean margins. Their argument, laid out in a series of papers over the past decade, is that the geological evidence for a fully desiccated Mediterranean followed by a catastrophic refill is much weaker than the standard story suggests. Some of their specific claims are worth being clear about.
First, they argue that the Mediterranean may not have fully disconnected from the Atlantic during the Messinian salinity crisis. The evidence for an entirely dry basin — as opposed to a much reduced, extremely salty sea — is ambiguous. If a partial connection remained throughout, the drawdown was less extreme than the standard story requires, and the “refill” was correspondingly less catastrophic.
Second, they argue that the Gibraltar channel García-Castellanos and colleagues attribute to catastrophic flooding may have been formed instead by longer-term, more gradual erosion processes. The channel exists. What cut it is genuinely contested. There are erosion models that produce similar channel geometries under substantially less dramatic flow conditions.
Third, they note that some of the sedimentary evidence used to date the catastrophic refill is compatible with a slower, more gradual return of Atlantic water over thousands of years.
None of this makes the standard story impossible. It makes it substantially less certain than the popular version suggests.
Why the debate is difficult to settle
The specific difficulty of settling this argument is that the event, whichever version is closer to correct, happened five million years ago in a place that is now underwater.
There are no witnesses. There is no direct record. Everything geologists have to work with is inferred from the specific patterns in salt deposits, sediment layers, buried river canyons, and eroded rock formations. Each line of evidence has multiple possible interpretations. The question of whether the Mediterranean refilled catastrophically or gradually is not the kind of question that can be answered by finding one more piece of evidence. It requires working out which interpretation of the existing evidence is most consistent with the whole picture — and geologists who have spent their careers doing exactly that work do not currently agree.
This is a specific feature of how deep-time science actually works, and it is worth being honest about. The popular version of geology tends to present findings with more certainty than the underlying evidence supports. When a striking model like the megaflood captures public attention, the caveats and alternative interpretations usually do not travel with it. The public gets the three-kilometre waterfall. The scientific debate about whether that waterfall actually existed stays in the journals.
What is genuinely known
Stripped down to what is directly supported by the evidence, the picture looks like this.
The Mediterranean underwent an extreme geological event between roughly 5.96 and 5.33 million years ago. Its sea level dropped substantially. Its salinity increased dramatically. Vast quantities of salt precipitated onto its floor. When the event ended, marine conditions returned, and normal Mediterranean sea life eventually re-established itself.
Whether the ending involved a catastrophic megaflood with sea level rising ten metres per day, or a slower refill over thousands of years, or something in between, is currently a live scientific question. The evidence is compatible with more than one story. Which story is correct will probably take another decade of research to fully resolve.
What is certain is that something extraordinary happened. The Mediterranean is a genuinely peculiar sea — saltier than the Atlantic, still connected to it by only a single narrow strait, with an evaporation rate that would drain the entire basin in about a thousand years if the Atlantic connection were cut again. It exists, in its current form, because of a specific chain of geological events, some of which we understand well and some of which we are still trying to work out.
The story of what happened at Gibraltar five million years ago is not settled. It is being actively investigated, right now, by geologists who disagree with each other in serious and interesting ways. The definitive version has not yet been written.
