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.

One of the properties of seas that has only recently been discovered is related in a verse of the Qur'an as follows:

"He has let loose the two seas, converging together, with a barrier between them they do not break through." (The Qur'an, 55:19-20)

This property of the seas, that they come together yet do not mingle with one another at all, has only very recently been discovered by oceanographers. Because of the physical force called "surface tension", the waters of neighboring 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. (Davis, Richard A., Jr. 1972, Principles of Oceanography, Don Mills, Ontario, Addison-Wesley Publishing, s. 92-93.)

The interesting side to this is that during a period when people had no knowledge of physics, surface tension, or oceanography; this was revealed in the Qur'an.