About 5.6 million years ago, the Mediterranean Sea began to evaporate. The Strait of Gibraltar, the narrow connection between the Mediterranean and the Atlantic Ocean, closed for the last time during a slow tectonic shift between Africa and Europe. The basin lost more water to evaporation each year than it gained from rivers and rainfall, and over a period that may have been as short as a thousand years, the entire sea disappeared. What remained was a salt-floored desert basin, in places three to five kilometres below the surrounding sea level — equivalent to roughly two miles deep. The basin stayed in that state for approximately 600,000 years. Then the Atlantic returned. The event known as the Zanclean Flood, around 5.33 million years ago, may have been the largest flood in Earth’s history. It also remains, in 2026, one of the more actively contested events in geology.
How we know it happened
The evidence for the Messinian Salinity Crisis, as the desiccation period is technically called, came from deep-sea drilling. In 1970, the research vessel Glomar Challenger spent 54 days drilling 28 holes into the Mediterranean seafloor. The cores it brought up contained something unexpected: layered evaporite deposits, including halite (rock salt) and gypsum, in some places hundreds of metres to several kilometres thick. According to the Wikipedia overview of the Messinian Salinity Crisis, the total volume of salt deposited in the Mediterranean basin during the desiccation is estimated at over one million cubic kilometres — enough to account for roughly six percent of all the dissolved salt in the world’s oceans. The “salt giant” formed in this period is one of the largest salt deposits anywhere in the world.
The presence of these deposits in the deep Mediterranean basin is difficult to explain except by mass evaporation. Salt precipitates out of water only when the water becomes hypersaline, which requires significant evaporation relative to inflow. Layers of fossil soils, river-cut canyons in the basin walls, and the presence of terrestrial plant fossils in deep cores confirmed that what is now the deep Mediterranean had been, at minimum, a hot and largely dry landscape. The closure of the Strait of Gibraltar precursor around 5.96 million years ago set the process in motion, and a final closure around 5.6 million years ago tipped the system into nearly complete desiccation.
What it would have looked like
For approximately 600,000 years, the area where ships now sail between Marseille, Algiers, Naples and Athens was a landscape of salt flats, brackish lakes, and deep canyons cut by rivers descending from the surrounding continents. The Nile flowed into a basin three kilometres below sea level, eroding a canyon over a kilometre deep beneath what is now the river’s Egyptian course. The Rhône and the Po cut similar incisions through France and Italy. Land bridges may have temporarily connected Africa and Europe across the former seafloor, allowing terrestrial animals to migrate between the continents. The “Lago Mare” phase, late in the crisis, saw shallow brackish lakes expand across parts of the basin floor with a fauna resembling that of the modern Caspian Sea.
The dry Mediterranean basin was hot. With air pressure substantially higher than sea level — possibly 50 percent above modern atmospheric pressure at the deepest points — temperatures at the basin floor would have been correspondingly elevated. Estimates suggest that some parts of the former seafloor could have reached temperatures comparable to or exceeding those of the modern Lut Desert in Iran, which has recorded some of the highest surface temperatures ever measured on Earth.
The refilling, and the dispute
The end of the Messinian Salinity Crisis is the part of the story where the science is genuinely active. The conventional account, established by a 2009 paper in Nature by Daniel Garcia-Castellanos and colleagues, is that the Strait of Gibraltar reopened around 5.33 million years ago and the Atlantic refilled the Mediterranean in a catastrophic event called the Zanclean Flood. The team’s modelling suggested that 90 percent of the basin refilled within a period ranging from a few months to two years, with peak discharge rates around 100 million cubic metres per second — roughly a thousand times the flow of the modern Amazon River — and sea level rising in the Mediterranean at more than 10 metres per day. The drop from Atlantic level to the deep basin floor was roughly 1,000 metres at the peak of the event — although more recent geological studies suggest the flow descended as a gradual high-speed channel rather than a true vertical waterfall, the popular “Gibraltar waterfall” framing has become the conventional way to describe it.
A December 2024 study by Aaron Micallef of the Monterey Bay Aquarium Research Institute, summarised in ScienceDaily’s January 2025 coverage, provided additional evidence supporting the catastrophic interpretation. The team identified more than 300 asymmetric, streamlined erosional ridges across south-east Sicily, aligned with the megaflood’s direction, plus a 20-kilometre-wide erosional shelf channel connecting the ridges with the Noto Canyon in the eastern Mediterranean. The features are interpreted as the erosional scar of the floodwaters as they spilled across the Sicily Sill from the western to the eastern Mediterranean basin. The lead author called the Zanclean megaflood “an awe-inspiring natural phenomenon, with discharge rates and flow velocities dwarfing any other known floods in Earth’s history.”
The catastrophic interpretation is, however, increasingly contested. According to a February 2026 review in Knowable Magazine, fresh doubts have arisen about nearly every element of the standard story: how dry the basin actually got, how isolated it was from the Atlantic during the crisis, and whether the refilling was genuinely catastrophic or a more prolonged process. Several recent papers have argued that the Mediterranean may never have been fully dry, that partial connections to the Atlantic persisted through much of the crisis, and that the Zanclean refilling could have unfolded over thousands of years rather than months. The catastrophic-flood version, which became conventional wisdom in the 1970s and was popularised by a David Attenborough documentary and a commemorative Gibraltar postage stamp, may not survive the next decade of geological research in its current form.
What remains settled
What no one disputes is the basic geological record. The salt deposits are real and enormous. The eroded canyons cut by rivers into the dried basin floor are real and have been mapped in detail beneath modern sediment cover. The Mediterranean was, at minimum, severely reduced in volume for hundreds of thousands of years, and the basin was substantially below sea level during that period. The interesting and contested question is whether the system tipped into total desiccation followed by abrupt catastrophic refilling, or whether the desiccation was incomplete and the refilling more prolonged.
Either way, the Mediterranean as it currently exists — saltier than the Atlantic, with restricted exchange through the narrow Strait of Gibraltar, prone to evaporation in its warm climate — is a sea that has demonstrably been close to drying out and refilling at least once in its geological history. The basin floor still holds the salt giant that the evaporation deposited. The eroded river canyons beneath the modern seafloor still trace the courses the rivers cut when there was nothing in the basin for them to flow into. Whether the refilling that ended the crisis was a months-long catastrophe or a millennia-long restoration, the Mediterranean is one of the few bodies of water on Earth whose existence has been demonstrably interrupted within the time that mammals have walked the planet.
