- In Surah 18:86, the Qu'ran says that a traveller sees the sun sets in a spring of murky water, and in 18:90 this same traveller finds the specific point at which the sun rises. We know, of course, that the sun does not set into a murky spring of water, and further that the earth is not flat, which is presupposed by the finding of specific places where it rises and sets.
Until, when he reached the setting of the sun, he found it set in a spring of murky water: Near it he found a People: We said: "O Zul-qarnain! (thou hast authority,) either to punish them, or to treat them with kindness." (18:86)
This verse refers to an area that is near to an ocean in the west. Thus, it did really look as if the sun sets in the water.
Until, when he came to the rising of the sun, he found it rising on a people for whom We had provided no covering protection against the sun. (18:90)
This verse refers to an area in the east.
Quran never mentioned the earth is flat. All it says is God has spread out the earth using the word spread which means to (cause to) cover, reach or have an effect on a wider or increasing area (Cambridge Advanced Learner's Dictionary) which explains the vastness or spaciousness of His creation.
And it is He who spread out the earth, and set thereon mountains standing firm and (flowing) rivers: and fruit of every kind He made in pairs, two and two: He draweth the night as a veil o'er the Day. Behold, verily in these things there are signs for those who consider! (13:3)