The Gateway to the Mediterranean
The Mediterranean Sea is a vast sea, almost entirely enclosed by land, with Europe to its north and Africa to its south. Yet it is not completely cut off from the world's oceans. There is one narrow natural opening that links it to the Atlantic Ocean, and that opening is the Strait of Gibraltar.
A Narrow Meeting of Continents
The Strait of Gibraltar is surprisingly narrow. At its tightest point, only a short distance separates the two sides, so narrow that you can clearly see one continent from the other. On the northern side lies Europe, with Spain and the famous Rock of Gibraltar. On the southern side lies Africa, with Morocco. The strait is the natural boundary between these two continents at this point, with the two landmasses almost touching.
The Only Natural Link
What makes the Strait of Gibraltar so important is that it is the only natural connection between the Mediterranean Sea and the open Atlantic Ocean. Without it, the Mediterranean would be a completely closed sea. Through this narrow channel, ocean water constantly flows in and out, and ships travel between the Mediterranean ports and the rest of the world. This makes the strait one of the busiest and most strategic waterways anywhere.
Two Currents in One Strait
The Strait of Gibraltar has a fascinating feature. The water of the Mediterranean is saltier and heavier than the water of the Atlantic, partly because so much of it evaporates under the strong sun. As a result, two currents flow through the strait at the same time, one above the other. Lighter Atlantic water flows in along the surface, while heavier, saltier Mediterranean water flows out underneath it. It is a hidden, two-layered river within the strait.
Source
This article was written using information from Wikipedia.