"We will show them Our Signs in the universe, and in their ownselves, until it becomes manifest to them that this (the Qur'an) is the truth. Is it not sufficient in regard to your Lord that He is a Witness over all things?"

[Fussilat, 53]

Western scientists discovered lately that there is a barrier between every two salty seas and this phenomenon was captured on cameras from spaceships.

This barrier prevents the water of the first sea to be mixed with the water of the second sea, thus no sea transgresses the other. Every sea has its own density, degree of saltiness and components.
This barrier is not steady, for it might move according to the wind speed and the ebb and flow of the tide.
These scientists were informed after this discovery that this fact is mentioned in the noble Quran in the following Ayah:

“He has let loosed the two seas (the salt water and the sweet) meeting together. Between them is a barrier which none of them can transgress. Then which of the Blessings of your Lord will you both (jinns and men) deny?” 

[Ar-Rahman, 19-21] 

They were greatly amazed as regards the Barrier mentioned in the above Quranic Ayah.
They also discovered that there is a barrier between the salty sea and the sweet water which prevents one of them to transgress the other.
Like in the barrier between two salty seas, the barrier between the salty water and the sweet water prevents the fishes of both waters from crossing the barrier. This breakthrough is also mentioned in the noble Quran in which the first kind of barrier is named "Barzakh" and the second barrier is named "Hijr".

Allah says:

"And it is He Who has let free the two seas (kinds of water), one palatable and sweet, and the other salt and bitter, and He has set a barrier and a complete partition between them."

[Al-Furqan, 53] 

As for the nature of this barrier, it is still subject of research.