The Mediterranean is a closed basin. There are rivers that flow in; there is the manmade Suez Canal, there is flow from the Bosporus and the Black Sea, but aside from Suez none of those link to the open ocean, and Suez wasn’t there 100 years ago.
The only connection to the open ocean is the Strait of Gibraltar. Through that passage, the Mediterranean exchanges its water with the open ocean, leading to strong currents across it. The Sea itself is actually fairly salty; more water evaporates from it than flows into it, building up the salt content with time. The only way the salinity of the Mediterranean doesn’t keep building up is that the water flows in and out at Gibraltar.
Picture for a moment then…the Strait of Gibraltar closed. It’s only 15 kilometers across and less than a kilometer deep. Close that gap and the Mediterranean becomes a bathtub with no drain and no faucet. Suddenly no water flows in from the ocean, no saline water flows out. Evaporation keeps going, rivers flow in, but they cannot keep up with evaporation and the sea itself starts to drain. As evaporation continues eventually, the whole sea would start to empty. The only thing keeping water in the Mediterranean Sea would be the trickle of water coming in from rivers, rather than the flood of water coming from the ocean.
About 5-6 million years ago, during the Miocene…something like this actually happened. It’s called the Messinian Salinity Crisis (why it’s a crisis I’ve never heard, maybe someone will explain it in a comment), named for salt mines near the city of Messina in Sicily.
This image, taken near Sorbas (Almería, España (Spain)), is one of the end results of the closing of the Strait of Gibraltar. This image shows one of many thick deposits of the mineral gypsum which surround the Mediterranean (note the house created out of the blocks for scale). Gypsum is a mineral made of calcium and sulfate; it’s actually the first abundant mineral created when ocean water dries up. Halite (sodium chloride) is more abundant, but gypsum comes out first.
Surrounding the Mediterranean, there are thick layers of this gypsum; more than could be created if the entire Sea dried up. They were formed over hundreds of thousands of years and surround the basin. The only way this much gypsum could be made is if the channel to the open ocean kept opening and closing perhaps dozens of times, or at the very least was restricted somehow. The Mediterranean grew saltier as the connection closed until the point solid gypsum deposits began to form, and as additional ocean water trickled in, more gypsum precipitated.
Later still, the Strait of Gibraltar closed even more, allowing the Mediterranean to eventually start to empty. Thick deposits of halite, including those at Messina, are found throughout the region as well, including in the sediments at the bottom (where the salt layer can be detected seismically). This salt precipitated when the Mediterranean Sea was more closed off and almost completely emptied; its depth was at least 2 kilometers below the modern day sea level. The salt layer in the Mediterranean again is so thick that it would make up about 10% of the salt in the entire Earth’s ocean.
How did this happen? Well, there are many theories. Sea level would need to fall substantially, more than we’ve seen from any glaciation in modern times, to cut off the Mediterranean, but that could be possible. Alternatively, something could have uplifted the land beneath Gibraltar enough to cut off supply. What tectonics accomplished this feat are not well understood, and it’s also not understood how the basin could open and close repeatedly to let additional water in to evaporate without opening the gap wider (some research on that subject found at the links below).
One thing that must have happened, however, is that the Mediterranean, reduced to a saline lake 2 kilometers below the normal ocean surface, must have refilled. At some point, whatever dam formed at the Strait of Gibraltar was breached. Breaching of a dam like this by the open ocean cannot have been a slow process; once the ocean started flowing in, it would cause erosion that would increase the flow rate.
In other words, around 5 million years ago…there was a torrent of water pouring through a channel and dropping a depth of 2 kilometers as the entire ocean poured in and refilled the Mediterranean Sea. The flow coming through this gap might have been 1000x that of the modern day Amazon River; truly a geologically epic event that unfortunately happened more than 5 million years before any human could have been around to witness it.
Image credit: Daniel Garcia-Castellanos, reproduced here with permission
Details on the refilling: