Around five million years ago, the Strait of Gibraltar closed and the Mediterranean dried up. When it reopened, the sea refilled in less than two years.
Geological terms are pretty great sometimes. The time, more than five million years ago in which it is thought that the Mediterrean was blocked off and subsequently dropped, is called the Messinian salinity crisis. The sea probably didn’t disappear completely. Instead, it would have been reduced to a few hyper-salty pockets of water in the bottom of a deep dry basin. And by deep, I mean several kilometres below sea level.
How do we know this? There are a few geological markers like salt deposits, but the biggest clue is the fact that rivers draining into the Mediterranean carved out large canyons that are now (in the case of the Nile, for example) two and a half kilometres underwater.
The Messinian salinity crisis ended about 5.3 million years ago when the Strait of Gibraltar reopened and waters from the Atlantic came rushing back in. We don’t know precisely how fast they returned, but current estimates suggest that it happened very quickly. Like, in a matter of months… perhaps two years. At its peak, that would be a volume one thousand times that of the Amazon, all rushing in to recreate the Mediterranean Sea.
This event is known poetically as the Zanclean deluge (or the Zanclean flood). It’s not a certainty – some scientists think that it happened slowly over ten thousand years rather than in such a rush. But I like the idea of 100,000,000 cubic metres of water per second pouring into the dry basin of the Mediterranean. To give you an idea of the scale, that’s enough to completely fill up the Grand Canyon in less than half a day. Damn.