This idea was also proposed by Alec Smith (Nature, 1985) but although the sea-floor topography as shown on hydrographic maps was tantalisingly consistent, it seems that Gupta et al. have been able to use much higher resolution modern sidescan sonar data to confirm the hypothesis.

Another interesting, and even larger, megaflood occurred through the Straits of Gibraltar at the end of the Messinian Salinity Event. Around 5.96 million years ago, uplift across the Pillars of Hercules and/or a fall in global sea-level led to the isolation of the Mediterranean basin from the world ocean.

Over a surprisingly short time (perhaps less than 1,000 years) the high temperatures in the region led to a progressive desiccation of the basin and the widespread deposition of ‘Messinian’ evaporite deposits across the basin. The distribution of these across the area led Hsu (1983) to proclaim ‘The Mediterranean was a desert’.

The presence on the basin floor of several cycles of evaporites record that this process was repeated multiple times, and at its lowest level, the Mediterranean salt lake surface may have fallen to at least 1 000 m below that of the North Atlantic, and, possibly to as much as 3 and 5 km below that of the North Atlantic.

We are all familiar with the climatic extremes of Death Valley, which lies only 86 m below modern sea level – in comparison calculations suggest that surface temperatures at the Messinian basin floor may have been as much as 30 to 50 degrees C higher than on the surrounding land, which was already warm and certainly would have become hotter and drier still following desiccation of the Mediterranean Sea.

Such a pronounced drop in base level resulted in massive erosion in all of the river systems flowing into the basin – seismic studies show the presence of a 2 400 m deep canyon beneath modern Cairo, for example.

Eventually, at around 5.33 million years ago, full connection with the North Atlantic was restored and the Mediterranean was filled again, probably by means of a single catastrophic flood through the Straits of Gibraltar. This was achieved via a huge waterfall up to 1 000 m high, which would have refilled the basin in as little as 3 months (see this whimsical picture to gain an impression of how such a waterfall might have looked, had humans been around to marvel at it.

An introductory review of the Messinian Salinity Crisis can be found on Wikipedia and more information is available by searching the many abstracts of scientific papers and geological lecture summaries to be found on Google.