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On the island of Sicily near the straits of Messina are immense deposits of evaporites, minerals that form when they drop out of solution from evaporating water.

In the 1970s, geologists using seismic profiling to perform a geological survey of the Mediterranean basin found a curious phenomenon:  A highly-reflective layer (dubbed the "M Reflector") buried beneath the sea's floor.

Later, the Glomar Challenger, sent to drill cores from the seabed found the astonishing cause of the reflector:  Directly underneath sea-floor sediments in the deepest part of the Mediterranean was a mixture of gravel and crystals of several minerals associated with evaporating seawater, notably anhydrite, which changes into gypsum in the presence of water.   Further investigation revealed that these deposits were 6000 feet (nearly 2 km) thick.  The cores also revealed evidence of stromatolites, which only grow in sunlight.

It was realized that the Mediterranean has a negative moisture balance: Although rainfall and rivers add 1830 km3 to the Mediterranean every year, it loses 4690 km3 every year to evaporation.   The balance has to be made up from the Atlantic Ocean. If water were not constantly flowing in through the Straits of Gibraltar, the Mediterranean Sea would dry up in 1500 years.

Finally, excavation in the Rhone and Nile river beds revealed that those rivers flowed through mile-deep gorges, since filled in with sediment.  The Nile's gorge caused the Soviet Union much trouble while building the Aswan High Dam, 1200 miles from the sea!

All of this evidence drew investigators to the astonishing conclusion:  Beginning 7-12 million years ago, the opening between the Mediterranean and the Atlantic was blocked by tectonic uplift from the African Plate pushing into the Eurasian plate, combined with a drastic drop in sea-level caused by an Ice Age.

The sea dried up over a period of 1000 years.

About 5-5.5 million years ago, tectonic shifts created a new opening between the empty basin and the Atlantic Ocean.  An unimaginably large waterfall 2600 feet (~1 km) high allowed the Atlantic to pour in and refill the Mediterranean over the course of 100 years.

Climactic consequences abounded:  The Mediterranean Sea floor was a desert 10,000 feet below sea level.  The final stages of the sea's evaporation would have looked like the Dead Sea recreated on a vastly larger scale. The part of Europe that wasn't covered by ice was a dry steppe.

Not only that, about 6% of the Earth's salt deposits were now locked up under the Mediterranean sea floor: Today's oceans are somewhat fresher than the oceans of millions of years ago.  Fresher water means a higher freezing point, and makes the formation of sea ice easier, and more difficult to melt once it has formed.  This "Messinian Salinity Crisis" led to global cooling, and a cascade of mass exctinction and evolutionary adaptation that had earlier led geologists to divide the Miocene epoch of the Cenozoic Era from the Pliocene epoch.

The Mediterranean's drying up has in turn led to the cycle of Ice Ages that dominate Earth's climatological history to this day.
 

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