In Surah Ar-Rahman (55:19-20), the Quran declares: "He released the two seas, meeting [side by side]; between them is a barrier [so] neither of them transgresses."
This verse describes an oceanographic phenomenon that modern science only discovered in the 20th century: the existence of natural boundaries between different water masses that meet without mixing.
The famous French oceanographer Jacques-Yves Cousteau studied this phenomenon. When the waters of the Mediterranean Sea meet those of the Atlantic Ocean at the Strait of Gibraltar, they do not mix immediately. An invisible barrier separates them, each retaining its characteristics (temperature, salinity, density).
This phenomenon is due to the difference in density between water masses. The Mediterranean water, saltier and denser, plunges beneath the Atlantic water, creating a natural stratification.
The same phenomenon is observed where the waters of the Red Sea meet those of the Indian Ocean, and in many other places around the globe where currents of different salinity or temperature meet.
The Quran also mentions in another verse (25:53) that between the two seas there is "a barrier and a forbidden partition," further confirming this oceanographic phenomenon.