"He has let loose the two seas, converging together, with a barrier between them they do not break through". (Qur'an, 55:19-20)
This property of the seas, that is, that they meet and yet do not intermix, has only very recently been discovered by oceanographers. Because of the physical force called "surface tension," the waters of neighbouring seas do not mix. Caused by the difference in the density of their waters, surface tension prevents them from mingling with one another, just as if a thin wall were between them. There are large waves, strong currents, and tides in the Mediterranean Sea and the Atlantic Ocean. Mediterranean Sea water enters the Atlantic by Gibraltar. But their temperature, salinity, and densities do not change, because of the barrier that separates them.
Modified from: www.miraclesofthequran.com