In Quran 25:53, the separation between two seas with different salinities is mentioned:

This verse describes a barrier (or barzakh) between two bodies of water—one fresh and pure, and the other salty and bitter. Critics might argue that such a description is mistaken, suggesting that these seas would mix into a single, averaged salinity. However, modern scientific research has confirmed the existence of a halocline, a phenomenon where bodies of water with different salinities remain separated by a boundary due to differences in their density, preventing them from mixing easily.

In oceanography, a halocline refers to a layer in a body of water where there is a significant difference in salinity. This difference in salinity, combined with temperature variations, creates a vertical stratification where less saline water sits above more saline water. This stratification can act as a physical barrier to the mixing of these water layers. The halocline is one type of chemocline that occurs due to the salinity gradient. This phenomenon was discovered only recently, with oceanographers observing such layers, particularly in places like the Arctic, where a halocline exists as a 150-meter thick barrier between freshwater and saltwater. This is a known scientific concept today but was accurately described in the Quran 1400 years ago.

The Quran’s description of the barrier between two seas with different salinities, and the term barzakh (a barrier), aligns with the scientific understanding of the halocline. The verse in the Quran implies that there is a separation between two seas due to their differing properties, a phenomenon now understood as water stratification caused by salinity differences.

The Quran’s mention of a barrier between two seas with differing salinities mirrors modern scientific knowledge of the halocline, a phenomenon that separates water layers based on salinity. This insight, accurate long before its scientific discovery, raises the question of how an unlettered man in the 7th century could have known such details. Many view this as evidence of the Quran’s divine origin, offering knowledge that was not accessible through human observation alone at that time. Whilst these verses might not necessarily reference the scientific findings mentioned here, they showcase how, even the most hard to imagine realities explained by the Quran, can be shown to be permissible with modern scientific discoveries.