Ancient Mediterranean reshaped by a colossal flood
An interdisciplinary study led by researchers at the Australian National University examines a dramatic event in the Mediterranean’s distant past. About five million years ago, the region witnessed a colossal flood that played a key role in shaping the basin into the active marine ecosystem seen today. The researchers describe a massive, 1.5 kilometer high waterfall near what is now Sicily that funneled vast amounts of seawater into the eastern Mediterranean, driving a sweeping hydrological transformation.
Known as the Zanclean Mega-Flood, this event stands as one of the most extreme floods documented in Earth history. It changed a desolate, landlocked saltwater expanse into a thriving sea, providing crucial insight into how the basin acquired its current ecological and geological character. The study highlights this flood as a turning point in Mediterranean history, offering a window into long term environmental change. The work has been featured in Nature Geology as part of the growing body of evidence on this transformative episode.
Today, shells have been found at high elevations on the Troodos Mountains, the island’s largest mountain range. The presence of marine remains far inland underscores a history dominated by tectonic shifts, rising and sinking land masses, and dramatic flooding that left lasting marks across the region.
Udara Amarathunga, the lead author and a PhD researcher in paleoenvironments at ANU, notes that the Zanclean flood is among the most abrupt global environmental shifts since the mass extinction that ended the era of dinosaurs. He describes the event as a Mediterranean rebirth, a watershed moment in the sea’s long evolution.
Gibraltar’s doorway and the closing of a sea passage
The mega-flood helped seal the end of the Messinian Salinity Crisis as the Atlantic–Mediterranean gateway near Gibraltar narrowed and eventually closed, leaving vast salt deposits and a drastic drop in life forms. Cosmos magazine summarizes this phase as a turning point that effectively left large portions of the basin saltier and less hospitable before the eventual reintroduction of marine water from the Atlantic.
In the present discussion, the Straits of Gibraltar are described as a closing door that did not snap shut overnight. Access to the Mediterranean was gradually restricted, reaching a near complete seal about five and a half million years ago. The resulting salinity crisis is viewed as the peak of a long sequence of tectonic and oceanographic events.
The research notes that while the exact extent of the drying remains uncertain, the basin likely split into eastern and western basins, separated by land features such as Sicily. This division helps explain dramatic changes in salinity and biogeochemical conditions across the region.
A giant waterfall and the flood dynamics
So what caused the Mediterranean to shift from a harsh saltwater body to a receptive marine environment again?
Amarathunga explains that gradual erosion at the Gibraltar gateway permitted small volumes of Atlantic water to breach into the Mediterranean. The mega-flood concept, initially proposed by researchers including Daniel García-Castellanos in 2009, suggests that a progressive flow eventually overwhelmed a natural barrier, triggering a colossal influx that created a massive waterfall centered near Sicily and inundated the eastern half of the basin.
During the peak of the flood, the energy released by the rapid water movement would have dwarfed modern benchmarks. At the height of the event, water levels were projected to rise dramatically, reshaping coastlines across the basin. If measured in energetic terms, the flood would have delivered a surge comparable to many times the annual output of famous waterfalls elsewhere, signifying an extraordinary geophysical episode.
The narrative continues with a striking image of the Mediterranean rising in level while global sea height declined. This swift, complex reorganization of water masses would have altered chemical conditions, oxygen distribution, and the formation of organic-rich layers in the water column—evidence pointing to short windows of anoxic conditions in the eastern basin after the flood.
The sequence of events likely unfolded in stages: filling the western basin first, followed by a land barrier breach near Sicily that produced an eastern waterfall. This cascade could have transported significant salt into the eastern basin, with later models indicating a protracted phase in which the salt was gradually redistributed toward the Atlantic, a process estimated to take tens of thousands of years before the Mediterranean basin stabilized anew as a single sea.
According to Amarathunga, this protracted transition period was previously unknown to scientists. While some debate persists about whether the Mediterranean ever completely dried, the ANU study adds weight to the mega-flood hypothesis by illustrating a rapid, system-wide transformation with lasting ecological and chemical consequences.
Amarathunga emphasizes that such a rapid and extensive shift is rare, making the Zanclean flood a unique example of how swiftly entire ecosystems can reorganize in response to dramatic hydrographic change.
As part of ongoing research, the study cites Nature Geology as a reference for the megaflood model and its interpretation in this context. This line of inquiry continues to shed light on how ancient hydrological catastrophes contributed to the modern configuration of Mediterranean seas.