A flood of biblical proportions filled the present-day Mediterranean Sea in a matter of months. At its peak the mega-flood caused the sea's level to rise by over 10 metres per day.

Around 5.6 million years ago the Mediterranean Sea almost completely evaporated when it became disconnected from the Atlantic Ocean. This was due to uplift of the Strait of Gibraltar by tectonic activity, combined with a drop in sea level. Further tectonic activity 5.3 million years ago lowered the Strait and reconnected the dry Mediterranean basin with the Atlantic.

Previously it was assumed that it took at least a decade to refill the Mediterranean, via a waterfall over the Strait. Now a new analysis shows that the bulk of the water arrived in less than two years, pouring down a long, shallow ramp.

Daniel Garcia-Castellanos of the Jaume Almera Institute of Earth Sciences in Barcelona, Spain, and colleagues studied borehole and seismic data from the Strait of Gibraltar to estimate the size and extent of the channel carved out by the flood. This revealed a gorge 200 kilometres long and 250 metres deep, cutting through the Strait. Unlike previous models, they reconstructed the rock layers that the water would have had to cut through, and how the channel would have evolved as a result.

They found the flood most likely started with a trickle of water over a hard rock barrier, taking thousands of years. This gradually deepened the channel until the barrier failed and water surged through, filling the Mediterranean Sea in less than two years (Nature, DOI: 10.1038/nature08555). The team estimates the peak flow to have been around 1000 times higher than the present Amazon river at its highest rate.

Flora and fauna had to adapt to the new environment rapidly and for some species, including the first hominins, the deep channel may have acted as a barrier. "If the land connection had remained it certainly could have facilitated an earlier arrival of early humans in western Europe," says Chris Stringer, an anthropologist from the Natural History Museum in London. Instead early humans took a circuitous route to western Europe and didn't arrive until 1.5 million years ago.