And it is He who has released [simultaneously] the two seas, one fresh and sweet and one salty and bitter, and He placed between them a barrier and prohibiting partition (25:53)
The fact that the waters of the oceans do not mix with each other has only been discovered lately by oceanographers. This is due to the physical force called “surface tension,” wherein waters of neighboring seas do not mix. It is 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. The Amazon River’s water pours into the Atlantic Ocean and yet preserves its traits even after it goes out 200 meters into the ocean.
Or [they are] like layers of darkness within an unfathomable sea which is covered by waves, upon which are waves, over which are clouds – layers of darkness, some of them upon others. When one puts out his hand [therein], he can hardly see it. And he to whom Allah has not granted light - for him there is no light (24:40)
It is well-known and affirmed scientifically that at extreme depths of the ocean there is no light, only darkness. The cause for this is that sunrays cannot reach it, for oceans range from being a few hundred meters deep to being 10994 meters deep. The darkness in the deep seas and oceans is found to begin at around a depth of 200 meters and below. At this depth, there is almost no light. Below a depth of 1000 meters there is absolutely no light penetration. Therefore, most of the sun’s rays are absorbed by the water at 100 meters. This portion of the sea is known as the luminous portion. Thereafter 1% of the sun’s rays are visible at 150 meters, and 0.01% of the rays at 200 meters.
The ozone layer in the atmosphere reflects most of the ultra- violet rays, whereas the clouds reflect 30% and absorb 19% of these rays. Only 51% of these rays reach the sea. Between 3% and 30 % of the sunlight is reflected at the sea surface. Then almost all of the seven colors of the light spectrum are absorbed one after another in the first 200 meters, except the blue wavelength.
Scientists have also recently discovered that there are internal waves which “occur on density interfaces between layers of different densities.” The internal waves cover the deep waters of seas and oceans because the deep waters have a higher density than the waters above them. Internal waves act like surface waves. They can also break, just like surface waves. Internal waves cannot be seen by the human eye, but they can be detected by studying temperature or salinity changes at a given location.
For example, the Mediterranean Sea’s water is warm, saline, and less dense, compared to the Atlantic Ocean’s water. When Mediterranean water enters the Atlantic over the Gibraltar sill, it moves several hundred kilometers into the Atlantic at a depth of about 1000 meters with its own warm, saline, and less dense characteristics. The Mediterranean water stabilizes at this depth. Although there are large waves, strong currents, and tides in these seas, they do not mix or transgress this barrier.
This information has only been known to us in this age, with the advanced technology and equipment available to detect these events. The Prophet lived in a desert climate far from the ocean, and he did not travel across any oceans. The fact that the Quran mentions this phenomenon indicates the veracity of the Quran.