All it took to get started was a trickle. Researchers have found evidence that the Atlantic Ocean refilled the dried-out Mediterranean basin about 5.3 million years ago in a gigantic flood, which started slowly but accomplished most of the task within as little as several months.
Today, the crystal-blue waters of the Mediterranean Sea cover some 2.5 million square kilometers. But 5.3 million years ago, that same area was a desert peppered by extremely salty lakes with depths down to 2.7 kilometers below current sea level. The sea's waters had evaporated about 300,000 years earlier, when the Mediterranean's only inlet at the time, the Strait of Gibraltar, blocked the flow of seawater from the Atlantic Ocean. Earlier research had suggested that a major ice age had caused sea levels to drop, exposing a land bridge between Europe and Africa.
Then, when the ice age ended, the sea rose again. Eventually, it breached the land bridge, creating what was called the Gibraltar Falls. Some scientists had described thousands of times more water flowing over the falls than at Niagara. But the evidence for this scenario has remained sketchy and the ice-age explanation has been questioned.
Now a team of researchers has provided a more-nuanced picture of the breaching of that natural dam: Instead of collapsing, it eroded over thousands of years. Daniel Garcia-Castellanos, a geophysicist of Institut de Ciències de la Terra Jaume Almera in Barcelona, part of the Spanish National Research Council, and colleagues reviewed all available data from boreholes and seismic studies and analyzed that data using models designed to simulate how fast-moving rivers can carve out channels.
Based on that analysis, the team concludes, the flood began as a relative trickle, as the land bridge subsided a bit. Over the next several thousand years, the rapidly growing stream carved a notch deeper and deeper into the land, until the inflow of seawater became overwhelming--containing possibly 1000 times more water than the Amazon River does today.
Although the flood built up gradually, it nevertheless grew to enormous proportions. Up to 90% of the refilling took place in less than 2 years--perhaps only a few months--raising the water level in the Mediterranean basin by more than 10 meters a day at its peak, the scientists argue  tomorrow in Nature. The action of that much water carved a 200-kilometer-long channel right through the center of the straits, deepening it by as much as 0.4 meter a day. “As soon as the first trickle of Atlantic water found the way through it, the feedback between erosion and water flow led to [that] enormous discharge in a short period," Garcia-Castellanos says.
The remaining mystery is the process that blocked the Mediterranean in the first place and then refilled it. "Our models do not help much in knowing what brought on the topographic divide between both seas," Garcia-Castellanos admits, but he suspects the cause was subsidence of tectonic plates.
It's a "very convincing" bit of research, says geologist Philip Gibbard of the University of Cambridge in the United Kingdom. The idea of a gentle influx growing into a gigantic torrent seems "perfectly reasonable." But to be sure, Gibbard suggests testing the erosion model further, by conducting lab experiments or perhaps applying it to data related to similar refillings of other basins in the geological record, such as the Black Sea.