Until when he reached the place where the sun set, he found it going down into a black sea, and found by it a people. We said: O Zulqarnain! either give them a chastisement or do them a benefit [Quran 18:86]
The verse says that Zulqarnain saw the sun setting into a black sea, which is the same expression that we would use to describe our experience. Once I traveled to San Diego, spent the whole day on the beach. When I got back, I told my friends that this was the first time that I had seen the sun setting into the Pacific Ocean, which is the same that Quran is reporting about Zulqarnain’s
experience. The verse doesn’t say that the sun goes down into a black sea. It simply says that it was Zulqarnain who found the Sun setting into a black sea. Verse 18:90 says:”[18.90] Until when he reached the land of the rising of the sun, he found it rising on a people to whom We had given no shelter from It.” Still talking about Zulqarnain, the Quran is describing his experiences, not what the scientific reality is. In our own times, Japan is called the Land of the Rising Sun.
If the Quran actually meant that the sun sets into a pond, why is it no companion of Prophet Muhammad ever tried to find that place?