The Camarinal Sill is a curious geological feature that lies twenty kilometers to the east of the narrowest point of the Straits of Gibraltar. It's an underwater rise that at its top is three hundred meters from the surface, with (much) deeper water on either side.
So the tips of the pincers forming the Straits also has a third pincer coming from below. And to make things ever more interesting, the northern and southern points are on separate tectonic plates -- the Eurasian Plate to the north, the African Plate to the south. These two plates have different relative motions, which is why around six million years ago, the straits abruptly closed up, and for a time, there was dry land between what are now Spain and Morocco.
The problem is, the region around the Mediterranean Sea is hot and dry (and was back then, too). With the Atlantic Ocean now cut off, the only inflows of water into the Mediterranean came from all the rivers draining into it. But the sum total of all that water entering it was still exceeded by the evaporation rate from the parched air passing over it.
Over the next six hundred thousand years, the sea level dropped by several kilometers, leaving behind a desiccated desert and a few widely separated lakes of concentrated brine. The temperatures in the region rose by an estimated 15 C year-round, creating a climate more like the central Sahara than the pleasant "Mediterranean climate" that places like Italy, Greece, Spain, and southern France now enjoy. The minerals from the evaporated sea water were left behind, creating layers of salt, gypsum, aragonite, and calcite that can still be seen today.
Then, 5.33 million years ago, there was another tectonic shift, and the two sides of the Camarinal Sill pulled apart.
At that point there was a five-kilometer difference in the sea level between the Atlantic and what was left of the Mediterranean Sea; in fact, from the shore of the Atlantic west of Gibraltar to the nearest Mediterranean brine lake was a distance of over three hundred kilometers.
The result was a flood to end all floods.
The "Zanclean Deluge" was so enormous it's hard to visualize. The waterfall over the newly-created Straits of Gibraltar was so powerful it eroded a nine-hundred-meter-deep gorge in the seabed. The water level rose by an estimated ten meters a day for a year, ultimately refilling the entire Mediterranean Basin.
[Nota bene: before any biblical literalists @ me with comments like "Ha-ha, that proves the Great Flood in the Book of Genesis actually happened!", allow me to point out that (1) the Zanclean Deluge had nothing to do with forty days and nights of rain, and (2) 5.33 million years ago our ancestors were small-brained hominins called Ardipithecus that lived in what is now Ethiopia, Tanzania, and Kenya. There is, I might add, no evidence that Ardipithecus could build giant boats, nor were they capable of going to Australia to fetch a pair of kangaroos so they could be saved from the Flood and bringing them back after God finished smiting the absolute shit out of everyone and the waters receded. Oh, and (3), don't you people disbelieve in plate tectonics anyhow?]
Anyhow, the reason this all comes up is that a team of geologists from Utrecht University, the Royal Holloway University in London, and the University of Granada have just found the first unequivocal direct evidence that all this happened -- a deposit of gypsum, sandstone, and marl (lime-rich silt) showing distinct ripple marks from flowing water. Upon analysis, they showed that the water was traveling really fast (something that can be determined from the wavelength of the ripple marks and the size distribution of the particles), was moving from west to east, and -- the clincher -- the entire formation dated to right around 5.33 million years ago.
Smoking gun, that."Now, for the first time, we can directly prove and quantify one of the most catastrophic periods of environmental change on our planet, which until now we had only been able to describe in geophysical models," said Gils Van Dijk, who led the study. "Moreover, geologists are trained to use contemporary processes on the Earth’s surface to interpret what we observe in rocks. But here we can’t rely on that knowledge, because we don’t know of any similar phenomena from at least the past one hundred million years."
There rolls the wave where grew the tree.O Earth, what changes hast thou seen?There where the long road roars has beenThe stillness of the central sea.The hills are shadows, and they flowFrom form to form, and nothing stands,They melt like mists, the solid lands --Like clouds, they shape themselves, and go.